The purpose of the Collision Countermeasures Systems (CCS) Task is to analyze the roadside infrastructure and traffic impacts of the following CCS:
1. Roadway-mounted friction/ice detection and warning systems.
2. Cooperative warning of the presence of oncoming vehicles on curves.
3. Driver warning on a minor road in the presence of vehicles on a major road.
4. Driver warning on a major road in the presence of vehicles on a minor road.
5. Approaching vehicle warning for drivers making a left-hand turn and warning of vehicles turning left ahead.
The purpose of Phase I of the CCS Task is to collect information on the sensor technologies available or being developed that can be used in the above countermeasures systems, to describe various deployment concepts, and to evaluate the various concepts' impacts on the existing traffic flow and infrastructure.
This document is a summary of the work performed in Phase 1 of this task. Section 2 of this report describes the survey, Section 3 describes the different types of sensors, and Section 4 describes the different deployment concepts. Section 5 discusses the traffic and infrastructure impacts of the deployment concepts described in Section 4, and Section 6 contains conclusions and recommendations.
The survey began with an extensive literature search. The first topic searched was collision countermeasure systems, but as this topic did not yield much information, the more general topics of detectors, sensors, and radars were searched. Relevant technologies for sensors had already been identified as being in the categories of microwave, millimeter wave, and laser radar, along with ultrasonic, piezoelectric, infrared, video, and inductive loop detectors. The literature on these subjects was searched to determine the most up-to-date developments in the field and to determine which technologies would be most useful for the collision countermeasure systems evaluated in this report. During the literature search, another technology which may be useful for collision countermeasure systems was discovered. It is a spread-spectrum wideband radar and will be discussed in Section 3.2.8.
The telephone survey consisted of determining which manufacturers produced sensors which could be used in the proposed collision countermeasure systems and then requesting data sheets on them. The results of this survey were then to be used to compare the different technologies from a performance and cost benefits perspective. The sensor manufacturers were also asked to supply their opinions on the tradeoffs between the different types of sensor technologies.
Various Departments of Transportation (DOT's) with experience in the relevant collision countermeasure systems were interviewed to determine the results of their efforts. This ensured that work would not be duplicated and that future work would build on the results and experience of past research. Their opinions on the different advantages and disadvantages of the various sensor technologies were also discussed.
Research institutions with relevant experience were interviewed to ensure that previous work on the collision countermeasure systems to be evaluated in this report could be incorporated and time would not be wasted reproducing existing results.
The survey of auto manufacturers was limited by the fact that not many have ever investigated any roadway-based collision countermeasure systems. However, at least one of the manufacturers (Nissan in coordination with MITI) had implemented one of the systems to be evaluated in this report and a paper describing the system was acquired.
A large amount of information was collected through the various surveys described in this section. In addition to that information, a report entitled "Potential Safety Applications of Advanced Technology"1 was provided by the Federal Highway Administration (FHWA), and a working report2 on a project being conducted by Hughes was obtained from Pete Mills of FHWA. The safety report, Reference 1, described many different types of sensor technologies, as well as several collision countermeasure deployment concepts. The Hughes working report, Reference 2, is a very detailed analysis of sensor technologies, and will include the results of extensive testing of the sensors both in the lab and in real world environments. The final data are currently being reduced.
Three different types of sensors are needed for the four collision countermeasure systems to be discussed in this report. They are: friction/ice detectors, vehicle detectors, and speed/acceleration detectors. Table 1 contains the different technologies that can be used for the different collision countermeasure systems.
TABLE 1
SENSOR TECHNOLOGIES AND APPLICATIONS
|
REQUIRED FUNCTIONALITY |
APPLICABLE SENSORS |
APPLICABLE COUNTERMEASURE SYSTEMS |
|---|---|---|
|
FRICTION/ICE DETECTION |
ROADWAY WEATHER INFORMATION SYSTEMS |
I |
|
VEHICLE DETECTION |
MICROWAVE MILLIMETER WAVE LASER INFRARED VIDEO ULTRASONICS PIEZOELECTRICS SPREAD-SPECTRUM |
II, III, IV, V |
|
SPEED / ACCELERATION DETECTION |
MICROWAVE MILLIMETER WAVE LASER INFRARED VIDEO ULTRASONICS PIEZOELECTRICS SPREAD-SPECTRUM |
V |
I. FRICTION/ICE DETECTION AND WARNING SYSTEMS
II. COOPERATIVE WARNING OF THE PRESENCE OF ONCOMING VEHICLES
ON CURVES
III. DRIVER WARNING ON A MINOR ROAD IN THE PRESENCE OF VEHICLES
ON A MAJOR ROAD
IV. DRIVER WARNING ON A MAJOR ROAD IN THE PRESENCE OF VEHICLES
ON A MINOR ROAD
V. APPROACHING VEHICLE WARNING FOR DRIVERS MAKING A LEFT-HAND TURN AND WARNING OF VEHICLES TURNING LEFT AHEAD
A friction/ice detector generally refers to the in-pavement sensors used to detect the freezing point at the surface of the roadway, the concentration of chemicals on the surface, and wetness of the surface. The in-pavement sensors are part of a larger roadway weather information system. Many of the pavement sensors measure the surface conditions by measuring the capacitance between two conductors which are almost flush with the surface of the roadway. Moisture on the roadway causes the capacitance to change, and thus the moisture and the concentration of chemicals within it can be estimated by the change in capacitance. The roadway weather information system then combines this information with data from the other sensors about the humidity, dew point, wind, temperature, and other factors to make an estimate of the roadway's present and future condition. Some pavement sensors have several sets of conductors for measuring capacitances. One of the sets of conductors may be heated incrementally. If the sensor originally indicated that the surface was dry, but after heating, it appears wet, the sensor will indicate that there is ice on the surface, and can provide a good estimate of the freezing point at the surface.
The Michigan Department of Transportation has investigated the use of a roadway weather information system built by Surface Systems, Inc. (SSI) for the prediction of preferential icing. Preferential icing describes the case in which ice appears on a bridge before it appears on the roadway. The sensors have proven to be only 20% accurate for this specific case. Therefore, they would not be useful as part of an active warning system for preferential icing. However, they may still be useful for the prediction of normal icing conditions and as part of an active warning system for drivers. An evaluation of the SSI system is contained in Reference 3. Some other systems for pavement surface detection are the Climatronics FRENSOR, the Vaisala DRS12, and AANDERAA Instruments. The Surface Systems, Inc. weather information systems seem to be one of the most widely used systems.
There is a wide range of sensor technologies available for vehicle detectors. Some of the most common and some developing technologies are described in this section.
A video image processor (VIP) is a combination of hardware and software which extracts desired information from data provided by an imaging sensor. This imaging sensor can be a conventional TV camera or an infrared camera. A VIP can detect speed, occupancy, count, and presence. Because the VIP produces an image of several lanes, there is potential for a VIP to provide a wealth of traffic information such as vehicle classification and incident detection. A VIP generally operates in the following manner: the operator selects several vehicle detection zones within the field of view (FOV) of the camera. Image processing algorithms are then applied in real time to these zones in order to extract the desired information, such as vehicle speed or occupancy.
Advantages of VIPs are that they are mounted above the road instead of in the road, the placement of vehicle detection zones can be made by the operator, the shape of the detection zones can be programmed for specific applications, and the system can be used to track vehicles. Disadvantages are the need to overcome detection artifacts caused by shadows, weather, and reflections from the roadway surface. The disadvantages can be overcome through design and installation of the hardware and design of the software algorithms.
The Hughes Report will contain the results of field tests on the Econolite Autoscope 2003, the Computer Recognition Systems Traffic Analysis System, the Golden River Traffic Marksman C-CATS 810, and the Sumitomo IDET-100. Table 2 details the specifications for these VIPS. Laboratory tests have not yet been performed on these systems. The field test results should be useful in quantifying the accuracy and performance of these systems in adverse weather conditions and differing roadway environments. The results may also be used to compare VIPs with inductive loop detectors.
TABLE 2
VIDEO IMAGE PROCESSOR CHARACTERISTICS
|
Detector |
Number of Traffic Lanes Monitored |
Speed Measure-ment Range |
Speed Measure-ment Accuracy |
Detection Range |
Vehicle Tracking |
|---|---|---|---|---|---|
|
AutoScope 2003 |
3 |
0 to > 80 mph |
± 2 mph |
46 m (150 ft) |
No |
|
Computer Recognition Systems, Traffic Analysis System |
3 |
0 to > 80 mph |
± 2 to 5 % |
46 m (150 ft) |
Yes |
|
Condition Monitoring Systems, Mobilizer |
3 |
0 to > 80 mph |
Ñ |
46 m (150 ft) |
Yes |
There are two types of infrared (IR) detectors, active and passive. Active infrared sensors operate by transmitting energy from either a light emitting diode (LED) or a laser diode. An LED is used for a non-imaging active IR detector, and a laser diode is used for an imaging active IR detector. In both types of detectors the LED or laser diode illuminates the target, and the reflected energy is focused onto a detector consisting of a pixel or an array of pixels. The measured data is then processed using various signal processing algorithms to extract the desired information. Active IR detectors provide count, presence, speed, and occupancy data in both night and day operation. The laser diode type can also be used for vehicle classification because it provides vehicle profile and shape data. The specifications for the Schwartz Electro-Optics 780D1000 active infrared radar are contained in Table 3.
TABLE 3
ACTIVE INFRARED DETECTOR SPECIFICATIONS
|
Detector |
Instan-taneous Field of View |
Vehicle Classi-fication |
Speed Measure-ment Range |
Detection Range |
Response Time |
Flow |
Presence Hold Time |
|---|---|---|---|---|---|---|---|
|
Schwartz Electro-Optics 780D1000 |
• 2 beams, each 1 mrad (El) by 9.5 deg (Az) • Beam separation in El = 10 deg |
Auto or truck |
0 to > 80 mph with ±1 mph accuracy up to 70 mph |
1.5 - 15 m (5 - 49 ft) |
~ 10 ms |
0 to > 1800 veh/h |
For as long as vehicle is in FOV of detector |
A passive infrared system detects energy emitted by objects in the field of view and may use signal processing algorithms to extract the desired information. It does not emit any energy of its own for the purposes of detection. Passive infrared systems can detect presence, occupancy, and count. The specifications for the Eltec passive radar systems are listed in Table 4.
Some of the advantages of infrared detectors are that they can be operated during both day and night, and they can be mounted in both side and overhead configurations. Disadvantages are that infrared detectors can be sensitive to inclement weather conditions and ambient light. The choice of detector materials and construction of the system, as well as sophisticated signal processing algorithms, can compensate for the disadvantages.
TABLE 4
PASSIVE INFRARED DETECTOR SPECIFICATIONS
|
Detector |
Detectable Objects |
Detection Range |
Response Time |
Maximum Speed at which vehicles are counted |
Hold time |
|---|---|---|---|---|---|
|
Eltec 842 |
Bicycle and any motorized vehicle |
6.4 - 16 m (21 - 54 ft.) |
< 500 ms |
45 mph |
True presence detector with 6 minutes maximum hold time for vehicles in FOV of detector |
|
Eltec 833 |
Bicycle and any motorized vehicle |
6.4 - 16 m (21 - 54 ft.) |
< 500 ms |
85 mph |
Pulse-type counting operation with count held for up to 4 seconds |
The Hughes report will contain the results of laboratory tests of the Schwartz Electro-Optics infrared laser radar detector 780D1000. The laboratory test results will include detector electrical current draw, delay time, engagement range, disengagement range, beam pattern, and operational and functional anomalies. The field tests are designed to quantify the accuracy and performance of the detectors in real-world environments. The data taken by the detectors being tested will be compared with "ground truth", measurements taken with inductive loop detectors, radar guns, and a video camera. Based on this comparison, the accuracy of the detectors will be quantified. There are plans to field test passive IR radars as well if they become available. The passive radars to be evaluated are the Eltec Passive IR Detectors 842 and 833. They were not available for laboratory testing, but are supposed to be tested in the field.
Ultrasonic detectors have not become widely used in the United States, but they are very widely used in Japan. Japan uses ultrasonic detectors in traffic applications as much as the U. S. uses inductive loop detectors in traffic applications. There are two types of ultrasonic sensors available, presence-only and speed-measuring. Both types operate by transmitting ultrasonic energy and measuring the energy reflected by the target. These measurements are processed to obtain measurements of vehicle presence, speed, and occupancy.
The advantages of ultrasonics are that they provide all-weather operation, do not need to be approved by the FCC, and provide fixed or portable mounting fixtures above the road. Their disadvantages include their need to be mounted in a down-looking configuration as perpendicular as possible to the target (as opposed to side-mounting), a difficulty in identifying lane-straddling vehicles and vehicles traveling side by side, and susceptibility to high wind speeds. Some of these disadvantages may be compensated for through more sophisticated data processing techniques.
Presented within the Hughes report will be the results of testing the Microwave Sensors TC-30 and TC-30C and the Sumitomo SDU200 and SDU300 ultrasonic sensors both in the laboratory and in the field. The TC-30C and the SDU300 are presence detectors, and the SDU200 also measures the vehicle's speed. The specifications of these detectors are contained in Table 5, which is taken from Reference 2. The SDU200 is designed to operate only with approaching traffic. The laboratory test results will provide information regarding detector electrical current draw, delay time, engagement range, disengagement range, beam pattern, and operational and functional anomalies. The field tests are designed to quantify the accuracy and performance of the detectors in real-world environments. The results will also be used to compare ultrasonic detectors with inductive loop detectors.
Microwave detectors have been used extensively in Europe, but not in the United States. They operate by measuring the energy reflected from target vehicles within the field of view. By processing the information received in the reflected energy, the detectors measure speed, occupancy, and presence.
Some of the advantages of microwave detectors are that they are a mature technology because of past military applications, they detect velocity directly, and a single detector can cover multiple lanes if it is placed properly and appropriate signal processing techniques are used. In addition, FCC approval is not required if it operates in the X-band or Ku-band, and the output powers are within specified limits. Some of the disadvantages are unwanted vehicle detections based on reception of sidelobe radiation, and
false detections due to multipath. Most of these disadvantages can be overcome, in whole or in part, through proper placement of the detectors, signal processing algorithms, and antenna design.
Within the Hughes report will be a presentation of the results of evaluating the Microwave Sensors TC-20, TC-26, and TC-31, the Whelen TDN-30, TDW-10, and Electronic Integrated Systems Remote Traffic Microwave Sensor (RTMS) in laboratory and field tests. Table 6, which was taken from Reference 2, lists some of the specifications for these radar systems. All of the radars operate in the X-band. The laboratory test results will include detector electrical current draw, delay time, engagement range, disengagement range, beam pattern, and operational and functional anomalies. The field tests are designed to quantify the accuracy and performance of the detectors in real-world environments. The data taken by the detectors being tested will be compared with "ground truth", measurements taken with inductive loop detectors, radar guns, and a video camera. Based on this comparison, the accuracy of the detectors will be quantified. 
Another type of vehicle detector is the passive acoustic array. An array of microphones may be used to determine the passage of a vehicle. The signals from the microphones in the array are processed and correlated to obtain information about vehicle passage. The design of the array determines its directionality and field of detection. These types of detectors have not yet been thoroughly investigated, at least in terms of traffic related applications. Video-conferencing companies have been developing sophisticated microphone arrays for their systems, and it is possible that some of their techniques or designs could be adapted to traffic applications.
The results of field tests of an AT&T SmartSonic TSS-1 acoustic detector array will be presented in the Hughes report. The results will be compared with those from the inductive loop detectors and magnetometers in order to judge its accuracy and level of performance.
Piezoelectric detectors are very accurate vehicle detectors, but they do not detect presence of a stationary vehicle, unless it has stopped with its wheels on the detector. The piezoelectric sensor consists of a long strip of piezoelectric material enclosed in a protective casing. It can be embedded flush with the pavement, and when a car passes over it, compressing the piezoelectric material, a voltage is produced. This sets off the controller. The piezoelectric detector has the advantage of indicating exactly when and where a vehicle passed by because it is a line detector perpendicular to the path of the vehicle. A series of two of them may be used to measure vehicle speed. A disadvantage is that for a permanent installation, they must be embedded in the pavement. Every time the roadway is repaved, or if a pothole appears, the sensor would need to be replaced. These types of sensors are not being tested in the Hughes report, but they are currently being tested on the Beltway in Virginia. AMP is a manufacturer of piezoelectric traffic detectors, and Table 7 contains some of the specifications for an AMP piezoelectric traffic sensor.
Photoelectric devices commonly consist of two components, the light source and the detector. These may both be in the same place, or placed across from each other. When placed across from each other, the detector is activated whenever something obstructs the illumination from the light source. When placed in the together, the detector is activated when light from the light source is reflected from a target and back onto the detector. There is a dearth of information on these detectors as applied to vehicle detection. They do not appear to be a competitive technology in the field of vehicle detectors at this time.
TABLE 7
PIEZOELECTRIC DETECTOR SPECIFICATIONS
|
Detector |
Output Uniformity |
Operating Temperature Range |
Temperature Sensitivity |
Output Level |
Product Life |
|---|---|---|---|---|---|
|
AMP ROADTRAX Series P |
± 20% |
-20Ð120¡F |
±0.2%/¡F |
50Ð1500 mV @ 1MW input impedance |
5 million axles typical |
A new wideband spread-spectrum radar has recently been developed at Lawrence Livermore Laboratory4. It is a significant development because it is very inexpensive and it has extremely accurate range discrimination. It can also penetrate many types of materials, including concrete. It has a range of about 20 feet, so it may be useful as an inexpensive, single-lane vehicle detector. It is predicted that the sensor, when made in production quantities, would cost much less than $10 per sensor. Because of their accurate range discrimination, they have a very well-defined field of detection. They could become a cheap alternative to magnetometer probes. Their ability to detect range provides additional information for future traffic control systems.
In addition, Lawrence Livermore has stated that they are developing a broad-band transmitter/receiver pair to be used with these sensors. This would eliminate the need for communication lines between the sensor and the controller.
Loop detectors are the most widely used technology for vehicle detection in the United States. A loop detector consists of one or more loops of wire embedded in the pavement and connected to a control box. The loop may be excited by a signal ranging in frequency from 10 KHz to 200 KHz. This loop forms an inductive element in combination with the control box. When a vehicle passes over or rests on the loop, the inductance of the loop is reduced. This causes a detection to be signaled in the control box.
The advantages of inductive loop detectors are that they are an established technology in the United States, they have a well-defined zone of detection, and they are generally reliable. Disadvantages are that the detectors are very sensitive to the installation process, they can only be installed in good pavement, and they must be reinstalled every time a road is repaved.
The Hughes report will contain only the results of field tests of inductive loop detectors. The loop detectors tested will be 3M microloops. The results will mainly be used as a point of comparison for the results from the other types of detectors, because inductive loop detectors are much like a standard in the United States. Sarasota is also a manufacturer of inductive loop detectors, and Table 8 displays some of their characteristics.
TABLE 8
CHARACTERISTICS OF INDUCTIVE LOOP DETECTORS
|
Detector |
Frequency |
Sensitivity |
Presence Time |
Inductance Range |
|---|---|---|---|---|
|
Sarasota 215B |
Provides frequency separation for adjacent loops |
3 ranges Maximum 0.02% change in inductance |
Maximum exceeds 1 hour for 1% change, 2 minutes for 0.05% change in loop inductance |
20 Ð 700 mH |
|
Sarasota 515A |
Provides frequency separation for adjacent loops |
3 ranges Maximum 0.02% change in inductance |
Preset to 2 hours, can be changed to 4 minutes |
20 Ð 2000 mH |
There are two other types of magnetic detectors which are used to detect traffic. Both of them are in the form of probes, and they both operate on the principle of a large metal object disturbing a magnetic field, just as inductive loop detectors work. There are both active and passive types. The active type is called a magnetometer. A magnetometer acts in much the same way as an inductive loop detector, except that it consists of a coil of wire wrapped around a magnetic core. It measures the change in the magnetic field caused by the passage of a vehicle. It can be used both for presence and for vehicle passage detection.
The passive type of detector simply measures a change in the flux of the earth's magnetic field caused by the passage of a vehicle. These detectors can only detect moving vehicles, so they cannot be used as presence detectors. They have a fairly large detection range and thus can be used to observe multiple lanes of traffic.
The advantage of both of these types of magnetic detectors is that they can be used where point or small-area location of a vehicle is necessary. For example, on a bridge, inductive loop detectors would be disrupted by the steel struts, and it is necessary to have a point detector. One of their disadvantages is that multiple detectors need to be installed to detect smaller vehicles, such as motorcycles.
The results of field tests of these detectors will be contained in the Hughes report. A Midian Electronics Self Powered Vehicle Detector will be evaluated and the results will be used along with the results from the inductive loop detector as a standard by which the aboveground detectors may be compared.
For the left-turn collision countermeasure system, it is necessary to determine the acceleration of the vehicle, so that it can be determined whether or not the vehicle is slowing to make a left turn. Using Doppler information, the range rate of a vehicle may be determined, but it does not appear that any radars currently being marketed for traffic applications measure the range rate. A simple method is to have three detectors in a linear formation. Measurements from these three detectors will provide an approximation of the acceleration of the vehicle from which the system may determine whether or not to activate the left-turn ahead warning.
The spread-spectrum wideband radar is a technology that could become established in the vehicle detector market. However, it is likely that inductive loop detectors (U. S.) and ultrasonic detectors (Japan) will continue to dominate the vehicle detector market and will remain the most popular form of vehicle detector technology. The final results of the Hughes report should be useful in making more accurate projections of which technologies will continue to be used and developed. Table 9 lists the majority of the sensors with their advantages and disadvantages.
This section provides a brief description of the different deployment concepts which were evaluated in this task.
This system should consist of a sensor system to detect the condition of the pavement surface and an active warning sign to provide a speed advisory. The sensor system should be implemented so that it measures the condition of the roadway surface at the point where the vehicle is most likely to drive. Most pavement sensors detect a very small, localized area, so they should be placed in the wheel tracks wherever possible in order to provide an estimate of the relevant surface conditions. A simple processor can then use the information about the condition of the pavement in combination with the known curvature, gradient, and dry-pavement friction coefficient to calculate an advisory speed. This speed would then be displayed on the active warning sign.
The processor should have some kind of limiter to prevent advising a speed greater than the maximum advisable for that area. It might be useful if it had a set of speed bins from which it could select an advisory speed. It may also be necessary to have a separate speed advisory for trucks. The normal difference in speed limits between cars and trucks is 10 mph. However, in the case of a slippery road, the truck is likely to retain more friction than a car, and thus its advisory speed may not have to be reduced as much as that for
TABLE 9
SENSOR ADVANTAGES AND DISADVANTAGES
|
SENSOR |
ADVANTAGES |
DISADVANTAGES |
|---|---|---|
|
MICROWAVE / MILLIMETER WAVE RADAR |
• Mature technology • Detect velocity directly • Multi-lane coverage • No FCC approval required if operating in X-band or Ku-band and power limits are met |
• Sidelobe radiation • Multipath |
|
INFRARED |
• Day and night operation • Overhead and side mount configurations |
• Sensitive to inclement weather • Sensitive to ambient light |
|
VIDEO IMAGE PROCESSOR |
• Roadway mounted • Variable detection zones • Vehicle tracking • Vehicle identification |
• Detection artifacts caused by shadows, weather, and reflections. |
|
ULTRASONIC |
• All-weather operation • No FCC approval necessary • Fixed or portable mounting |
• Mounting configuration needs to be perpendicular to target • Difficult identifying lane-straddling vehicles and vehicles traveling side by side. • Susceptible to high wind speeds |
|
PIEZOELECTRIC |
• Precise detection zone • Portable or fixed installation |
• Embedded in pavement |
|
SPREAD-SPECTRUM |
• Very inexpensive • Aboveground installation • Accurate range discrimination |
• Short range of detection |
|
INDUCTIVE LOOP DETECTOR |
• Mature technology • Well-defined detection zone • Accurate for counting |
• Embedded in pavement • Sensitive to installation process |
|
MAGNETOMETER |
• Small and useful for bridges where loops won't work • Portable or fixed installation |
• Embedded in pavement |
the car. This will have to be investigated more thoroughly before an actual advisory speed processor is designed.
Another possible implementation of a friction/ice detection and warning system would be to include some type of vehicle speed detector. Then, after the system makes an estimation of what the safe speed should be, it can choose whether or not to illuminate a sign saying "SLOW DOWN" based on the oncoming vehicle's speed. This would make it necessary to have a detector which measures speed, and thus adds to the complexity. However, radar sensors can detect both presence and speed, so it is possible that one radar could be used for both. Figure 1, obtained from Reference 1, illustrates the deployment concept.
The major equipment for use in this countermeasure system is: pavement sensors to cover as much of the pavement as possible and an active warning sign. This assumes that the system has access to a complete weather information system of which the pavement sensors are only a small part. The following is a summary of the potential friction/ice detection systems:
1. Use a pavement sensor to determine the condition of the pavement. Once the condition of the pavement has been determined, calculate an advisory speed and provide it to the warning sign. The sign will then determine what type of message to provide, i.e., "SLOW DOWN", or "BRIDGE MAY BE ICY," or the advisory speed.
2. Use a pavement sensor to determine the condition of the pavement. Define a class of "dangerous" conditions, and have a signal asserted if any of those conditions is met, such as icy, or wet. Supply this signal to the warning sign.
3. Same as 2, but generate an advisory speed for the dangerous condition and supply the speed to the warning sign controller.
4. Same as 1, but also use another detector to calculate vehicle speed. Supply this speed to the controller. Controller can then decide what type of warning, if any, to provide, based on a comparison of the advisory speed and the actual speed.
5. Same as 3, but also use another detector to detect vehicle speed and supply this data to the warning sign controller. Controller then decides what to do based on comparison of advisory speed and actual speed.
Figure 1. Road layout with an in-the-road friction detector.
A collision countermeasure system of this type is currently in operation in Japan. It has undergone extensive testing on a test track and has now been installed in actual portions of the highway. The name of the system is Guidelight. One of the Guidelight systems consists of a series of lights around the curve and an ultrasonic detector on each end of the curve. When a vehicle is detected, the lights are activated ahead of the vehicle at a rate dependent on the speed of the vehicle. The lights warn the driver of another vehicle entering the curve from the opposite direction that there is an oncoming vehicle. It is described in detail in References 5Ð8. The ISO standard being developed for "cooperative warning of the presence of oncoming vehicles on curves" is based upon the Guidelight system, so Guidelight may become the standard collision countermeasure system for this type of warning. Figure 2 shows an example of the Guidelight system, and is taken from Reference 1.
Activated by ultrasonic vehicle detectors
Figure 2. Schematic for Guidelight system on curves.
Another possible collision countermeasure system is proposed in Reference 1. It would consist of a pair of warning signs which would be activated as soon as a vehicle enters the curve in order to warn vehicles traveling in the opposite direction. A possible active warning sign would have two flashing lights on top and depict a two-way traffic road (assuming there are only two lanes) with a car in the oncoming lane. Both the flashing lights and the representation of the car will flash when the sign is activated. Figure 3 illustrates the deployment concept, and Figure 4 shows a possible active warning sign. Both figures are taken from Reference 1.
The major equipment for this countermeasure system is: vehicle detectors and a series of lights if using the Guidelight system or at least 2 warning signs if using the system described above. For hilly areas the sign could depict a straight lane with a car lighting up in the oncoming lane.
The following are possible deployment concepts:
1. In the simplest system, there should be at least 2 sensors and 2 signs. The two sensors are used to detect a vehicle entering the curve, and the active warning signs are placed inside the curve. This prevents the case of both cars entering at the same time and then passing the signs before they are activated.
2. Another option is to have four warning signs, two at the entrances to the curve and two along the curve. One set of signs should be set a good distance ahead of the curve on either side, in order to give the drivers enough advance warning that another car has entered the curve in the oncoming lane. The other set of signs should be set right within the curve so that cars that have passed the advance warning sign will still be notified if another car has just entered the curve.
Figure 3. Schematic of a limited sight curve with a single sensor-sign pair.
Figure 4. Warning sign on a limited sight curve.
This system is designed to enhance the driver's ability to assess the safety of entering an intersection on a major road from a minor road. There would need to be an active warning sign for the drivers on the minor road, and detectors to detect vehicles on the major road. A system of this type has already been implemented in Japan as part of the Guidelight program. A similar system is described in Reference 1.
A basic system would have two active warning signs, one on each approach to the major road. The signs should indicate not only that a car is approaching on the major road, but also from which direction.
There will also need to be as many detectors as there are lanes on the major road, and they will need to be a sufficient distance away such that the warning can be given in an adequate amount of time. The signs should be visible to the car on the minor road until he actually makes the turn. Thus, if it is in the position of most stop signs, it may not be visible as the vehicle prepares to make a turn, so there is the possibility that a vehicle appears right after the driver has moved passed the sign. In Japan, in a "T" intersection, they have placed the sign across the road, so there is no possibility of not being able to see it because of preparation for a turn. That may well be the optimum placement. The major equipment needed for this countermeasure system is: vehicle detectors for every lane on the major highway and at least one active warning sign. Figure 5 provides a detailed illustration of this deployment concept. Figure 5 was taken from Reference 1. 
Figure 5. Warning drivers on a minor road of the presence of vehicles on a major road.
This implementation will be similar to that for the previous collision countermeasure system except that it is the vehicles on the major road that will be warned. The detectors will need to be placed on the minor road sufficiently far back to provide adequate warning to the driver on the major road. If there is a stop sign at the intersection on the minor road, then a detector could probably be placed in the intersection and right before the stop sign. If there is only a yield sign, it may be appropriate to place the vehicle detector farther back along the minor road. The sensors in the middle of the intersection should remain in either case.
The detectors will provide information as to whether there is a vehicle on any of the minor roads, and whether or not there is a vehicle in the middle of the intersection. The detector in the middle of the intersection needs to discriminate between cars crossing the intersection from the side road and cars crossing with the flow of traffic. A variety of sensor configurations can accomplish this. One radar sensor can detect directionality, and two piezoelectric sensors could also determine directionality. A smart controller would combine the information from all of the detectors to determine where the vehicle that has entered the intersection has come from. The major equipment needed for this countermeasure is: vehicle detectors to detect the vehicles on the side roads and in the intersection, and at least 2 warning signs. Figure 6 illustrates the deployment concept, and it was taken from Reference 1. 
Figure 6. Warning drivers on a major road of the presence of vehicles on a minor road.
This system needs to perform multiple functions. First, it must identify that a vehicle is slowing down to make a left turn. It then needs to determine whether or not there is enough time to make the left turn based on the speed and location of oncoming traffic, and to activate an active warning sign appropriately.
The system must also activate a warning sign for vehicles following the driver making the left turn. An additional option is to have another sign to warn the oncoming traffic that a vehicle is making a left turn ahead. Sensors are needed to detect the acceleration of the vehicle that will be making the left turn, to detect the vehicle if it is still waiting to make a left turn, and to detect vehicles in the oncoming traffic lanes.
The most challenging aspect of this concept is to detect that a vehicle is slowing to turn left. Doppler radars can measure the range rate directly, whereas inductive loop detectors and spread-spectrum wideband sensors need to take multiple measurements and integrate them.
In an example multiple detector system for detecting the acceleration of a vehicle, a central controller would observe the timing between successive activations of the detectors. When the spacing increases above a certain threshold and indicates a predetermined amount of deceleration, the controller activates the left-turn ahead warning signal. The left-turn ahead signal will stay activated for a preset amount of time before turning off. If a speed threshold is used instead of an acceleration detector, the central controller should use memory of the most recent average speed so that the current speed can be checked against that. This would allow the system to adjust to changes in the flow of traffic. Figure 7 contains a schematic of a possible implementation of this deployment concept. It was taken from Reference 1.
The major equipment needed for this collision countermeasure system is: vehicle detectors to calculate acceleration and presence of vehicle waiting to turn left, vehicle detectors for the traffic in the oncoming lanes, one controller, and four active warning signs.
The following are three potential implementations of this collision countermeasure system:
1. A series of sensors can be set up to measure the acceleration of the vehicle. If it is decelerating at a rate greater than some threshold, then the left turn-ahead sign can be activated. In addition, there should be another sensor in the area where the vehicle would be turning left. If the sensor detects a stationary vehicle in this area, then it will also activate the left-turn ahead warning sign.
2. If congestion reaches high levels, then determining whether or not a car is slowing due to congestion or to make a left turn is more complicated. In this case, a sensor to detect slowing and a sensor to detect a stationary vehicle in the left turn position can be installed. The sensor which triggers based on a deceleration level can be deactivated in cases of heavy congestion, and so can the sensor which triggers on a stationary vehicle.
3. If there is a stop-light ahead of the left turn area, the same setup that is in example 1 can be used, but the information about the phase of the stop light should be used when deciding whether or not a car is decelerating to make a left turn.
Figure 7. Approaching vehicle warning for driver making a left-hand turn
and warning of vehicle turning left ahead.
The systems discussed in this report are safety oriented systems and apply only in very specific circumstances, so it is unlikely that they will have any significant traffic flow impacts. The main traffic impact should be a reduction in the number and severity of accidents. The infrastructure impacts will be much greater, especially in the areas of installation and maintenance of the systems, and liability.
An active friction/ice detection and warning system is a safety countermeasure system, so it is unlikely to impact the flow of normal traffic. Its main benefit is that it may reduce the number and severity of accidents occurring on icy roads.
The cost of the equipment and the cost of the maintenance of the system is prohibitive if used exclusively for a friction/detection system. The friction/ice detectors are really part of a road weather information system. If the sensors serve the double purpose of providing information to the road weather information system and to active warning signs, it becomes much more cost effective. Many of roadway weather information systems have been installed throughout the world. For example, about ten of these systems have been installed by the Michigan Department of Transportation. They cost about $60,000 per installation. In addition, they have 1 man spending about 80 percent of his time year round to maintain the installations. The costs of running the system are generally paid for by the savings in deicing chemicals and reduction in overtime payments due to more accurate predictions of the road surface conditions.
Any system designed to warn the driver of potentially dangerous pavement conditions must be very accurate to avoid the possibility of frequent lawsuits. Information obtained from Leo Defrain of the Michigan Department of Transportation indicates that using these systems to detect preferential icing on bridges is not reliable enough for an active warning system, though it may be accurate enough for active warning signs on normal roadways. Preferential icing on bridge decks is the situation in which ice forms on the deck of the bridge before it forms on the roadway. Leo Defrain has been studying detection systems for preferential icing on bridge decks at about 10 sites in Michigan. During the situations when it is essential for the system to be accurate, i.e. during preferential icing, it only operates correctly 20% of the time. This is unacceptable in any warning system because of the liability involved.
On the other hand, pavement conditions of a normal roadway are more uniform than in the preferential icing environment. The roadway weather information systems have proven to be successful in the prediction of roadway surface conditions in general and may be accurate enough for an active warning sign system.
A discussion of liability issues must really focus on the event of a real or perceived failure of the system from the driver's point of view. There are several ways in which the system could fail. It could indicate ice on the road when there wasn't, it could not indicate ice when there was, and if there were a power failure, it would indicate nothing. In any of these cases, the driver who is in an accident near one of these signs could easily sue the state.
Leo Defrain provided several examples of court cases involving road pavement conditions and the responsibility of the state in making the driver aware of them. In the first example, the types of signs used to warn of potential icing of bridges had to be changed. The original sign stated, "WATCH FOR ICE ON BRIDGE," but a driver who had an accident on the bridge sued the state based on the argument that a driver could not have watched for ice on the bridge and driven at the same time without getting in an accident. Therefore, the signs were changed to "BRIDGE MAY BE ICY".
In the second example, a major interchange that has a curve with limited sight distance in addition to being a bridge was the sight of many accidents. The bridge had two signs, one saying "LIMITED SIGHT DISTANCE, SLOW TO 45 MPH" and the other saying "BRIDGE MAY BE ICY". However, a Judge ruled that the state was responsible for warning the cars driving around the curve of stopped vehicles ahead, or else of proving that that was an impossible task.
In the third example, there was a 1.5 mile bridge with "BRIDGE MAY BE ICY" signs on either end. A driver had an accident in the middle of the bridge and said it was because he had forgotten that he was on a bridge. Therefore, the state had to put up three signs along the length of each side of the bridge to remind the driver that he was still on a bridge and that it still might be icy.
The last example again deals with a "BRIDGE MAY BE ICY" sign. It was argued that because the sign was up all year, drivers ignored it and thus could cause accidents on the bridge.
Clearly, the liability issues involved here must be fully evaluated before any of these systems are implemented. As a general rule, Mr. Defrain said that a reliability of at least 90% is desirable in any warning system. Considering the liability issues of this particular system, a reliability of 99% or greater would be best.
The above examples also indicate the importance of clearly conveying the warning to the driver through the sign, which is a human factors issue. The advent of active warning signs raises the issue of the average driver's assumption that when the warning sign is off, it guarantees safety. This is not the intended use of an active warning sign, but it is highly probable that it will often be misinterpreted in this manner. Thus, any active warning system will most likely need to err on the side of caution to ensure that there are no missed vehicle detections. This should minimize the number of lawsuits arising due to assumptions that the off state of an active warning sign guarantees safety.
This collision countermeasure system is intended to reduce the number of accidents around blind curves, so its only potential traffic impact is a change in the number of collisions around blind curves.
Both of these systems should be relatively inexpensive. The system described in this report may be cheaper because it does not require a sensor for speed and has fewer components than the Guidelight system. The Guidelight system has many more components to service because it consists of a string of lights around a curve, whereas the system described in this report only uses a maximum of four signs. The Guidelight system requires no cutting of the pavement because it uses ultrasonic sensors and the light elements are mounted along the guardrail. The other system may not require any cutting of the pavement either, depending on the type of sensor chosen.
This is the most important consideration. As always, there is the potential for a suit for every malfunction or perceived malfunction of the system. Interpretation of the meaning of the active warning signs in the one system and the lights in the Guidelight system could lead to some initial difficulties. However, tests on the Guidelight system indicate that most drivers were able to understand their use or at the very least, they did not misinterpret them in a such a way as to cause an accident.
When using active warning signs, it is important to clearly convey the warning to the driver. Unless the warning sign is very clear, the driver may think that only one car is permitted to enter the curve at a time, and thus he may stop and cause a rear-end collision. This should not happen once drivers become accustomed to the sign. Because of the many possible interpretations of a sign, the simpler implementation of lights in the Guidelight system may be best.
The goal of this collision countermeasure system is to reduce the number of accidents at intersections of a minor and major road. Thus, the traffic impact is hoped to be a reduction in the frequency and severity of accidents at these types of intersections. It should not have a significant effect on the traffic flow patterns unless widespread use in an urban environment is achieved.
The installation of this system will require the installation of vehicle detectors in every lane of the major road, and they will need to be connected to the active warning signs on the minor road. The elements of this system are well known, so there should not be any significant or unusually high costs for the implementation of this system.
In this collision countermeasure system it is particularly important that drivers do not interpret the unactivated signs as a guarantee that the intersection will be clear. The signs are meant to encourage the driver to look more carefully for oncoming traffic, not for the driver to blindly trust in the signs and to cross the intersection. However, someone will probably make the assumption that since the signs were not activated, the intersection was clear, and try to cross it and perhaps run into another vehicle. The probability of a missed detection in combination with a driver on a minor road crossing the intersection without looking anywhere except the signs should be estimated. It may be low enough to be tolerable. The government's responsibility for the drivers' safety in such a system should be investigated thoroughly, as well as the government's liability in the case of a malfunction of the system.
Visibility at the intersection can greatly affect the usefulness of these devices. If the driver on the minor road can see far enough in both directions on the major road, then the driver can easily discount a false alarm. In cases where visibility is at least somewhat restricted, false alarms become very important and need to be minimized, because the driver is putting his trust in the reliability of the sensors and the party responsible for the system could be liable for any errors. The warning signs are mainly designed to cause the driver to look again in cases where he may not have been very observant.
If drivers begin to take the fact of the signs in the inactive state as an indication that there is no oncoming traffic, then these signs would tend to increase the likelihood of an accident. For example, drivers who are in a hurry and who are approaching a major road from a minor road and who see that the sign is not activated may assume that the road is clear and attempt to cross it. If one of the sensors had failed, then there is potential for a fatal accident. Because some drivers depend on signs and not on their own powers of observation, when a sign fails, especially an active warning sign, many accidents could occur. When an active warning sign fails, it is not necessarily clear that it has failed, as in the case of a stop-light, so it can be a lot more dangerous.
This collision countermeasures system is also designed to reduce the number of accidents at the intersections of minor and major roads. It is not expected to affect traffic flow.
The installation and maintenance of this system should be straightforward. Vehicle detectors need to be installed on the minor roads and connected to the active warning signs on the major roads. Any maintenance required should be minimal as the systems will need to be highly reliable.
The liability issues in this countermeasure system are just as important as in the previous countermeasure system. In this case, a missed detection can result in the driver on the major road running into a driver from the minor road who has entered the intersection. Of course, this assumes a lack of visibility or the driver's assumption that a sign in the off-state guarantees that the intersection is clear.
This collision countermeasure system is designed to reduce the number of crossing-path accidents for vehicles making left turns. The warning of a vehicle waiting to turn left ahead is intended to make the drivers aware that they need to either slow down or change lanes. However, it probably will not impact traffic flow, because most drivers tend not to merge until necessary, especially in heavy traffic.
The system will need several sensors to be installed to detect vehicles and to detect acceleration, as well as a simple processor to calculate the acceleration and to decide whether or not the car is slowing to turn left. The costs of installation could vary depending on the type of highway involved. If there is a median, sensors could be installed in the median, minimizing the interruption of traffic flow for both installation and later maintenance. If there is no median and no overhead mounting area, sensors will need to be embedded in the pavement. This involves considerably more cost and a disruption of the flow of traffic.
Again, there is the problem of how an unactivated sign will be interpreted by the general driving public. A sign in the unactivated state does not guarantee that there is no oncoming traffic. This system and the previous system have the greatest potential for fatal accidents in that both of them are meant to aid drivers in crossing oncoming traffic. In both cases, if the driver depends exclusively on the signs, which would be an inappropriate use of them, he may cause a serious accident.
This report has presented an overview of available sensor technologies for friction/ice detection, vehicle detection, and acceleration detection. A description of the five collision countermeasure systems and a summary of possible traffic and infrastructure impacts has been presented as well. Information contained in this report included data from References 1 and 2.
For the friction/ice detection sensors and the cooperative warning around curves countermeasure systems, the following are recommended for Phase 2 of the CCS task:
• Monitor developments in implementation
• Monitor test results
• Monitor development of new sensor products
These recommendations are based on the fact that both friction/ice detection sensors and cooperative warning around curves countermeasure systems (Guidelight) have been implemented and are being tested in the field.
The rest of the CCS are mainly concerned with intersection warnings. For these CCS, the following are recommended for Phase 2 of the CCS task:
• Determine possible scenarios Ñ Use maps to determine locations of intersections where these CCS apply and use data from NHTSA to determine which intersections could benefit most from the CCS, based on the number of incidents.
• Develop system options Ñ Drawing from the deployment concepts presented in this report, develop candidate systems which meet the specific needs of each scenario. Compare systems based on different detector technologies, determining which technologies or combination of technologies provide the best system performance in each scenario.
• Perform first level cost analysis Ñ For each system, analyze the costs for installation and maintenance.
• Performance analysis Ñ Parameterize the performance of the proposed systems based on the differing sensor technologies. Incorporate data from the Hughes report when it becomes available.
• Recommend operational tests Ñ Recommend operational tests for the most likely systems. Possibly incorporate the recommended tests into existing operational tests.
A final recommendation for Phase 2 is that the spread spectrum sensors described in Section 3.2.8 be investigated for potential application as traffic sensors. However, prototypes of the sensor will not be available for experimentation until sometime in 1995, so it may not be possible to experiment with them before the completion of Phase 2.
From the Phase 2 report, it will be possible to determine, for a given intersection and CCS, the best combination of sensor technologies to apply, where and how to deploy them, and the costs for installation and maintenance of the CCS. The Phase 2 report will incorporate the results from Reference 1 when they become available, using them to help determine which sensor technologies best meet the needs of each scenario and CCS.
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