Wavetronix
For motorists, the safest possible condition with current intersection control strategies exists when a red light is already displayed as they approach an intersection. But what happens if the light changes as they approach the intersection?
The stop on red problem occurs if a motorist is about 2.5 to 5.5 seconds away from entering an intersection when the light turns to yellow. This creates a “decision dilemma zone” in which the driver must decide to speed through the intersection, and risk running the red light or right-angle collision; or stop suddenly and risk the more common rear-end collision. As illustrated in the following table, red light running, acceleration through the intersection and abruptly stopping are all major components of the stop on red problem.
| Component of Stop on Red Problem | Statistical Significance |
|---|---|
| Driver enters intersection after red | 15% of signalized crashes and 33% of deaths |
| Driver accelerates through intersection on yellow/red | 28% of signalized crashes are right-angle collisions |
| Driver abruptly stops on yellow/red | 42% of signalized crashes are rear-end collisions |
As stated before, the decision dilemma zone describes the area upstream of the intersection where the stop on red problem originates. It should not be confused with the classical definition of the physical dilemma zone, which exists when insufficient yellow time is programmed into the traffic controller, creating actual locations on the road where motorists, driving at or below the speed limit, physically cannot stop at, nor proceed safely and legally through, the intersection.
Classical analysis of the physical dilemma zone is sufficient to select a reasonable yellow time, but it ignores several fundamentals of the stop on red problem: motorists' expectation of the duration of the yellow light; motorists' estimated arrival time at the intersection; the presence and separation of multiple vehicles on the roadway; motorists' differing reaction capabilities; and the varying speeds at which different motorists travel.
The driver decision dilemma zone takes into account all of these factors, and it is important to note that the decision dilemma zone exists no matter what yellow time is programmed into the traffic controller. The following figure illustrates the different decision dilemma zones that exist for motorists driving at different speeds. In this example, the decision dilemma zone starts at 5.5 seconds and ends at 2.5 seconds from the stop bar. Notice how the dilemma zone is wider and further back for higher speed vehicles.
Programming correct yellow times can help, but this approach is really only effective if all motorists drive at the same speed with the same capabilities. Since most motorists drive at a range of speeds and react to things in different ways, there is not one “correct” or “safe” duration for the yellow light.
Generally-accepted yellow clearance intervals range from three to six seconds, but most drivers are unaware of the actual duration of the yellow clearance interval when they estimate their arrival time to the intersection in deciding whether to stop or go. Often motorists are caught off-guard by the duration of yellow and end up running a red light, accelerating through the intersection, or stopping abruptly. When other vehicles are involved, these conflicts frequently result in collisions.
The following table shows yellow clearance intervals calculated for intersection approaches with different design speeds using the “ITE equation.” The customary assumption is a deceleration rate of 11.2 feet per second squared (a=11.2 ft/s2) and 1.0 second reaction time.
| Yellow Clearance Interval (sec) | ||||
|---|---|---|---|---|
| a=10.0 ft/s2 | a=11.2 ft/s2 | a=12.5 ft/s2 | ||
| Speed (mph) | 75 | 6.5 | 5.9 | 5.4 |
| 70 | 6.1 | 5.6 | 5.1 | |
| 65 | 5.8 | 5.3 | 4.8 | |
| 60 | 5.4 | 4.9 | 4.5 | |
| 55 | 5.0 | 4.6 | 4.2 | |
| 50 | 4.7 | 4.3 | 3.9 | |
| 45 | 4.3 | 3.9 | 3.6 | |
| 40 | 3.9 | 3.6 | 3.3 | |
| 35 | 3.6 | 3.3 | 3.1 | |
| 30 | 3.2 | 3.0 | 2.8 | |
The SafeArrival technology developed by Wavetronix directly addresses the decision dilemma zone and the fundamental factors of the stop on red problem. SafeArrival technology detects the estimated time of arrival (ETA) of all vehicles approaching an intersection stop bar; if any vehicle's ETA is determined to be unsafe, then the technology signals the traffic controller to extend the green light.
The advance detection that SafeArrival technology provides offers the most effective protection of the decision dilemma zone. It works by simultaneously detecting the speed and range of vehicles every five milliseconds; it then divides each vehicle's range to the stop bar by its current velocity to determine that vehicle's ETA.
The power of SafeArrival technology is that it dynamically protects vehicles on the approach based upon their current speed, and not based upon the design speed of the approach. This also allows safe gaps to be found and reported instantaneously based upon arrival time, and not merely on the time between vehicles passing over discrete points of the roadway.
SafeArrival technology can only be found in the Wavetronix SmartSensor Advance™, a non-intrusive traffic detection device that uses a patented radar technology to detect the speed and range of vehicles approaching the intersection.
Safe Arrival Detection: In this picture, can you tell who's in the Dilemma Zone? SmartSensor Advance can, because it detects the arrival time, range and speed of vehicles within its 500-foot reach.
SafeArrival technology also provides focused protection of the arrival times that might be problematic at a particular intersection. Users can specify the minimum and maximum ETAs that warrant green extension; bounds on the speeds and ranges that are protected can also be customized to provide the most focused protection possible.
This customization is performed using the SmartSensor Manager ACE™ (Advance Compact Edition) software, which runs on laptops, Pocket PC®, or Windows® Mobile devices. The following screenshots illustrate the software's user interface and the tools available for customization.
In this example, the minimum ETA has been set to 2.5 seconds, and the maximum ETA has been set to 5.5 seconds. In addition, the minimum speed boundary has been set to 35 mph and the maximum speed boundary has been set to 100 mph. Finally, the range of protection has been constrained from 50 feet to 380 feet from the stop bar.
SafeArrival technology offers many benefits. First, it improves the efficiency and safety of intersections and reduces both intentional and unintentional red light running. Second, it operates in all weather and temperature conditions. And third, it provides additional benefits when used: with coordinated signals on arterial corridors; with advanced warning systems; with red light cameras; and to extend the red light.
Improves Efficiency, Safety, and Reduces Red Light Running. Intersection efficiency improves traffic flow, reduces the amount of time and fuel wasted as vehicles wait for the light to change, and helps to reduce harmful emissions. SafeArrival technology's advance detection provides the following efficiency benefits:
SafeArrival technology cycles control from under-saturated to saturated phases, using the added information of arrival time and speed to capitalize on the natural variances in traffic speeds and headways that occur, especially during under-saturated traffic flows. Thus, it terminates under-saturated flows by finding instantaneous gaps in traffic; after discharging the initial queue, SafeArrival technology can then cycle the light to the next phase sooner if traffic speeds drop below the lower speed bound.
Traditional protection technologies often sacrifice safety for efficiency, but SafeArrival technology improves both simultaneously. The safety benefits of SafeArrival technology can be enormous. A 10-year study of green extension systems like SafeArrival at high-speed signalized intersections found they reduced rear-end accidents by 75 percent, right-angle collisions by 31 percent, and red light running by 65 percent.
Operates in Harsh Conditions. SafeArrival technology has been engineered to operate on the most cost-effective radar platform available, which uses our patented Digital Wave Radar™ to provide consistently accurate detections in thousands of challenging highway locations. The performance of Digital Wave Radar is not affected by changes in temperature, weather or light, so SafeArrival technology continues to work accurately and reliably, regardless of the conditions.
Added Benefits. SafeArrival technology integrates easily with a variety of existing systems to provide a number of additional benefits:
Coordinated Signals. SafeArrival technology's decision dilemma zone protection reduces the number of collisions on arterial corridors with coordinated control of signals by operating traffic controllers in actuated coordinated mode. This mode continues to streamline the start of the mainline phases using offset, but it also uses a fully actuated means of terminating those phases and it promotes progression through each intersection, especially when spacing, priority and other practical issues come into play. For example, if the intersections on a corridor are unevenly spaced, coordination often favors one direction over another. But advance detection for actuated coordinated operation encourages two-way progression by dynamically widening the green band in the non-favored direction. SafeArrival's decision dilemma zone protection also effectively reduces collisions on corridors using traffic adaptive systems.
Advance Warning Systems. Advance warning systems (AWS) are becoming more popular, particularly at newly installed intersections, intersection with sight restrictions, and in rural areas where drivers may drive at high-speeds for long periods of time without encountering signalized intersections. In AWS, signs installed along the approach to an intersection warn drivers to “Prepare to Stop” seconds before the light turns yellow. SafeArrival technology improves the AWS by detecting safe gaps in traffic several seconds upstream from the decision dilemma zone; when the detected gap moves over the dilemma zone, SafeArrival first triggers the warning sign and then tells the controller it is safe to change the light to yellow. Typically, the onset of the AWS sign will lead the onset of the yellow light by three seconds or more, so the detection zones for dilemma zone protection must be several hundred feet upstream from the AWS sign. With Digital Wave Radar, SafeArrival technology has a detection range of 500 feet, so it is the perfect solution for dilemma zone protection in an advance warning system.
Red Light Cameras. Automated enforcement of red lights using cameras is reported to have a modest aggregate benefit of about $35,000 in societal costs per site per year. Primarily, this benefit results from a reduction in fatalities and severe injuries caused by right-angle accidents. Unfortunately, the use of cameras in automated enforcement does not reduce the most common type of signalized intersection collision — the rear-end crash. In fact, with the installation of a red light camera, an aggregate 15 percent increase in rear-end accidents can be expected. By integrating SafeArrival technology with red light cameras, the disadvantages of automated enforcement can be largely mitigated.
Red Light Extension. SafeArrival technology is flexible enough to provide either green light or red light extensions. Why would red light extension be warranted? Even with the decision dilemma zone protection offered by green extension, some drivers will still violate the red light, and the controller's programmed passage time will still “max out” in some cases. To further prevent collisions due to the clearance interval, SafeArrival can hold the red light in situations where a vehicle is detected to have a high likelihood of running the red light. Drivers that run the red light would still be issued a citation, but the red light extension would help prevent them from killing or injuring themselves or other motorists.
Vehicles rarely travel at the posted speed limit, and sometimes, circumstances require that the speed limit of a road be changed. Unlike traditional loop-based protection systems, SafeArrival technology's performance is not affected by these two facts. SafeArrival technology's decision dilemma zone protection is based on the actual speeds it detects on the roadway, and not on the design speed of a particular intersection. So when the posted speed limit or 85 percentile speed changes, or if motorists' speeds deviate from the design speed, the protection provided by SafeArrival technology will automatically adjust to the new distribution of speeds on the roadway. And because SafeArrival technology is individualized to the speed of motorists on the road, the motorists are still correctly protected even when they are not driving the speed limit.
Loop-based protection is not very effective when the actual speed of vehicles on the approach deviates from the design speed of the protection. For example, in a loop-based protection system set for a 55 mph approach, the green light is extended for at least three seconds whenever a vehicle passes over on the loops. This ensures that only those motorists driving at exactly the design speed will not be trapped in a 2.5 to 5.5 second decision dilemma zone. However, protection is mismatched for motorists driving faster than the design speed: they will be under-protected at far distances; and over-protected at near distances. The reverse mismatch problem is incurred by motorists driving slower than the design speed: they will be over-protected at far distances; and under-protected at near distances.
At the extreme, the protection for a vehicle traveling at 35 mph is completely misapplied. A common solution used to protect lower speed vehicles is additional extension time; however this increases overprotection for faster vehicles.
SafeArrival technology correctly resolves gaps and headways by instantaneously measuring the arrival time, speed and range of separate vehicles as they simultaneously approach the stop bar. By individualizing protection based on motorists' speeds, SafeArrival technology can find even more gaps, further increasing the efficiency benefits: in some cases, missing a small, safe gap in traffic could mean extending the green for 10 or more seconds on an under-saturated approach and eventually forcing “max out” of the green light.
Arrival time-based gap detection virtually widens each detected gap and allows instantaneous “gap out” when a vehicle leaves, or before it enters, its decision dilemma zone, instead of when the controller's detector or phase extension timer counts down to zero. In most cases, this means that SafeArrival technology will induce safe “gap out” one or more seconds before a similar loop-based system.
To understand why this is beneficial, consider the following four cases representing all possible types of gaps: a fast vehicle leading a fast vehicle; a fast vehicle leading a slow vehicle; a slow vehicle leading a slow vehicle; and a slow vehicle leading a fast vehicle.
Fast leads fast. When a fast vehicle leads a fast vehicle, loop-based systems have a difficult time detecting any gap between the two because they over-protects the first vehicle long after it's out of its decision dilemma zone; the trailing vehicle will also be over-protected when it exits its decision dilemma zone. SafeArrival technology will not over-protect either vehicle. If the arrival time gap between the two vehicles is sufficient, then the system will “gap out” at this point. If the gap between the two is not large enough, then the system will “gap out” as soon as the second vehicle leaves its decision dilemma zone, without an unnecessary delay due to over-protection.
Fast leads slow. When a fast vehicle leads a slow vehicle, a loop-based system will still have a relatively difficult time detecting any gap between the two since it overprotects the first vehicle. With SafeArrival technology, the first vehicle will not be over-protected and the second vehicle will not be under-protected: if the arrival time gap between the two vehicles is sufficient, then the system will “gap out” earlier; if the gap between the two is not large enough, then the system will “gap out” as soon as the second vehicle leaves its decision dilemma zone, without an unnecessary delay.
Slow leads slow. When a slow vehicle leads a slow vehicle, if the gap between the two is large enough, SafeArrival technology will “gap out” as soon as the first vehicle leaves its decision dilemma zone; otherwise, it will “gap out” when the second vehicle leaves its decision dilemma zone, without an unnecessary delay.
Slow leads fast. When a slow vehicle leads a fast vehicle, it is unlikely that the gap between the two is large enough to safely “gap out” the phase. However, SafeArrival technology will instantaneously “gap out” the phase as soon as both vehicles leave their decision dilemma zones, preventing unnecessary delay due to over-protection of the fast vehicle.
SafeArrival technology is an integrated dilemma zone protection system that matches protection to individual vehicles and is perfectly adapted to the stop on red problem. Loops, on the other hand, are point detectors that can be used with the passage time parameter of a traffic controller to provide a system somewhat adapted to the problem. Loops can only match protection to vehicles that drive to set parameters; vehicles which deviate from those parameters are either over- or under-protected, or missed altogether.
Both solutions strive to reduce accidents and red light running, but SafeArrival technology has several advantages over traditional loop-based green extension systems: greater accuracy and reliability; the ability to detect individual vehicle speeds; the ability to avoid the “max out” of green time; and the ability to detect instantaneous gaps.
The following figure offers a brief comparison of traditional, loop-based protection versus SafeArrival protection. The loop-based system in this example uses only two loops, one in each lane at a distance of 450 feet upstream of the intersection stop bar to provide advance detection.
Accuracy and Reliability. The Wavetronix SmartSensor Advance with SafeArrival technology and Digital Wave Radar is a non-intrusive device that installs above-ground and provides a 500-foot detection range. The consistent accuracy of Digital Wave Radar allows SafeArrival technology to detect each vehicle's speed and range as it approaches the intersection, and its broad view of the road allows one SmartSensor Advance to do the work of multiple loops. Loops must be buried in the road, a process that disrupts traffic and results in a 20 percent failure rate each year. Loops work with the passage time of the controller, which is usually only long enough to allow a vehicle to pass from one point detector to another, so if one loop fails, then the whole system fails. Multiple loops used in sequence can improve the accuracy of loop-based systems, but they also increase the number of potential failure points in the system. Even if the rate of missed detections for a single loop is just five percent, the overall system accuracy drops from 95 to 77 percent when five loops are used in sequence.
Individual Vehicle Speeds. SafeArrival technology constantly monitors the speed of individual vehicles and automatically adjusts protection according to the speeds it detects. This ensures that each motorist will be correctly protected even when they aren't driving the speed limit. Loop-based protection is most effective when vehicles travel over the loop and through the decision dilemma zone at the design speed; loop protection is misapplied whenever vehicle speeds deviate from the design speed. The misapplication of protection by loops not only reduces the effectiveness of single vehicle protection, it also reduces the probability that a safe gap in traffic will be detected for the active movements and dramatically increases the probability that loop-based systems will terminate the active phases in “max out.”
Avoiding “Max Out”. “Max out” occurs when the maximum green time is reached and the signal is forced to turn yellow regardless of the number of vehicles in the decision dilemma zone. This reduces the frequency of the hazardous clearance interval by dynamically maximizing the cycle length, but the loss of dilemma zone protection every cycle seriously raises the collision potential; and returns in efficiency diminish quickly because of the cumulative delay of the vehicles on the inactive movements. SafeArrival technology's ability to monitor each vehicle as it passes through the decision dilemma zone increases the safety and efficiency of intersections while avoiding “max out.” But loop-based systems “max out” frequently, in both peak traffic and under-saturated conditions, and the high-frequency of “max-out,” even during under-saturated traffic flow, is due to the misapplication of protection.
Instantaneous Gaps. SafeArrival technology correctly resolves gaps and headways by instantaneously measuring the arrival time, speed and range of separate vehicles as they simultaneously approach the stop bar. Loop-based protection cannot instantaneously measure gaps in traffic, or the time between vehicles; instead, loops typically require the passage of one or more seconds. Also, traditional loop-based protection does not use speed traps to directly determine the velocity of vehicles on the roadway, so the speed of each vehicle is not known. As a result, neither the distance between the vehicles nor the arrival time of vehicles to the stop bar can be detected. Furthermore, when multiple loops are used in multiple lanes, each detected time gap merely represents the period elapsed between two separate vehicles passing over any one of the loops.
Digital Wave Radar gives SafeArrival technology a broad, continuous view of the roadway and makes the consistent high resolution of speed and arrival time detection possible, even at distances two times beyond the reach of passive non-intrusive traffic technologies.
SmartSensor Advance has an extended reach of 500 feet, resulting in less trenching and a broad 400-foot view of the roadway. And since radar is a range-based technology, a 20-foot zone is resolved just as easily at 500 feet as at 50 feet, a notable advantage over video technology in which the numbers of pixels diminish at far ranges.
In addition, the broad beam of SmartSensor Advance is wide enough to monitor three lanes of traffic from 100 feet to 500 feet away from the sensor, and vehicles at the same range in different lanes will be automatically clustered together.
Continuous detection over the 400-foot range span provides 100 percent coverage, instead of only monitoring the traffic for a small percentage of the decision dilemma zone. The complete coverage of SmartSensor Advance has built-in redundancy when compared to detection methods using virtual point detectors. Furthermore, continuous detection overcomes single point failures by tenaciously holding on to detections even through dead spots in the background and instantaneous occlusion.
All versions of SmartSensor Advance are software upgradeable to use SafeArrival technology. The first software release of SmartSensor Manager™ with SafeArrival technology is dated 6.19.06. There is no charge to upgrade SmartSensor Advance to use this powerful new technology.
SmartSensor Advance has been designed to integrate easily with existing controllers as well as coordinated corridors, green extension, green extension with advance warning signs and red extension systems. They can also be used in place of loops or in conjunction with red light cameras. Contact Wavetronix for integration information for specific products and systems.
Existing Controllers. The SmartSensor Advance is compatible with standard 170, 2070, TS1, and TS2 traffic controllers. SmartSensor Advance signals a standard input file rack card, which then signals the traffic controller via contact closure, all of which are included in the system. Controllers such as the Econolite ASC/2, the Peek 3000 and the Naztec 981 can all be easily configured to work with the SmartSensor Advance.
Coordinated Corridors. Most modern controllers offer an actuated coordinated mode, although it is sometimes referred to by other names. The Econolite ASC/2 refers to it as “actuated coordinated”; the Eagle EPAC 300 Series has several modes which provide green extension, including “actuated coordinated,” “permissive yield,” “permissive omit,” and “sequential omit”; the Naztec 980 provides green extension through “coordinated yield” or “early yield”; and Peek controllers refers to it as “extended greenband percent.” Even older controllers may still be able to provide coordination of the mainline through phases and fully actuated termination. If the phase sequence is fixed, you can coordinate the phases directly previous to the mainline through phases, and then put the mainline through phases on minimum recall.
Green Extension Systems. The following figure shows one way three group alerts can be used to control the duration of green with standard traffic controllers.
| Group | Zones | Group Bounds | |
|---|---|---|---|
| Arrival Time | Speed | ||
| 1 (initial green) | 2 | None | None |
| 2 (flow speed) | 1 | None | >1, <35 mph |
| 2 (SafeArrival) | 1, 2, 3 | >2.5, <5.5 s | >35 mph |
Group 1 is used to increase the length of initial green beyond a 20-second minimum by detecting vehicles as they queue up on red. It is assigned to a detector input in the controller that only contributes to the added initial. This feature is currently available in many manufacturers' traffic controllers as specified by the NTCIP 1202 detector conformance group.
Group 2 is used to reduce the queue by holding the green light until vehicles on the approach are moving faster then some minimum speed range, or until the queue has already dissipated. It is assigned to a 1–2 second extend detector with the ability to disconnect the detector for the rest of the phase after the first falling-edge of the contact closure.
Group 3 is used to provide dilemma zone protection, in conjunction with volume-density operation of the phase. When using the gap reduction feature of volume-density mode on phases extended by SafeArrival technology, it is important to ensure that the initial passage time and minimum gap are set correctly. For example, if the protection has been selected for arrival times between 2.5 and 5.5 seconds from the stop bar, it already ensures a three second arrival gap in traffic. Instead of setting the initial passage time to six seconds and the minimum gap to three seconds as might be done with loop-based protection, the initial gap can be set to three seconds and the minimum gap can be set to 0 as shown below.
Green Extension With Advance Warning Signs. In most cases, SmartSensor Advance can be mounted on the same pole as the advance warning sign and configured with enough flash lead time to allow for instantaneous gap detection. The volume density mode can also be used to progressively favor protection of large vehicles; the sensor will then use its built-in range finder and occlusion to detect segments of the traffic flow not occupied by large vehicles. Over time, progressively smaller segments of traffic flow will become acceptable to the system in order to avoid “max out.”
Red Extension. A SmartSensor Advance group alert can be programmed to hold the red only for vehicles above a certain speed and with a certain arrival time. Newer controllers, like the Econolite ACS/3, already support red extend detector inputs, making dynamic red light extension readily available.
Loop Guidelines. The SmartSensor Advance can be used to implement an agency's loop-based dilemma zone guidelines. While the SmartSensor Advance's real power comes from SafeArrival's ability to protect large detection zones, SmartSensor Advance can easily be configured to protect smaller zones that produce the same type of detections as those generated by loop-based point detectors.
Red Light Cameras. The red light camera system works independently of the SmartSensor Advance, so mutual integration of the systems is not required. Simply integrate the SmartSensor Advance for green or red extension as desired.
Consider the approach shown in Figure 1. The left through-lane is protected with SafeArrival detection, while the right through-lane employs a multi-point detection method similar to those used with inductive loop detectors.
In this example, all vehicles are traveling the design speed of 50 mph. The point detector method allows for 2.0 seconds of passage time between P2 and P3 in an effort to protect vehicles traveling between 42 and 50 mph. The passage time is also applied after P3 while vehicles approach the intersection.
At the instant depicted in Figure 1, Car D has traveled 0.68 s beyond P3, leaving 1.32 s for Car C to reach P2, which it would do in only 0.75 s. Continuing at 50 mph, Car C would clear P3 after another 1.64 s. At that time, if no additional cars were following Car C, the phase would gap out after an additional 2.0 s, requiring a total of 4.39 seconds from the start of the example to gap out (timing summarized in Table 1).
Figure 1. A comparison of SafeArrival detection (left lane) and tradition point detection (right lane).
| Event | Delta Time (s) | Passage Time Remaining (s) | Time From Initial Conditions (s) |
|---|---|---|---|
| P3: Detect Car D | 0.0 | 1.32 | -0.68 |
| P2: Detect Car C | 0.75 | 2.0 | 0.75 |
| P3: Detect Car C | 1.64 | 2.0 | 2.39 |
| Gap detected | 2.0 | 0.0 | 4.39 |
In contrast, SafeArrival would detect the 3.0 s gap between Car A and Car B instantaneously. With no passage time to delay the gap detection, gap out would occur immediately, while giving the driver of Car A 5.5 s to make the decision to go or stop, and allowing Car B to clear the far end of the intersection before the onset of the red signal (timing summarized in Table 2).
| Event | Delta Time (s) | Passage Time Remaining (s) | Time From Initial Conditions (s) |
|---|---|---|---|
| Gap detected | 0.0 | 0.0 | 0.0 |
Not only did SafeArrival safely and efficiently shave 4.39 s off of the current phase, consider the additional delays incurred if Car C were followed by other vehicles that further extended the green signal. Every time a suitable gap is missed by a dilemma zone protection system, the likelihood of reaching a dangerous max out increases.
With the newest major release of the software, SmartSensor Advance provides the flexibility to configure multiple non-overlapping zones and then assign those to one or more groups that signal the traffic controller. In all, there are eight contact closure output groups that can be used to alert standard controllers of various traffic conditions. For example, a point detection zone can be configured and then assigned to a group alert that dynamically increases the initial green based upon demand.
As another example of additional traffic conditions that can be monitored, a group alert can be configured to extend the green until traffic is flowing above a specified speed. This group alert will make sure the phase is progressing adequately, before terminating the phase using SafeArrival technology.
By calling an inactive phase before the vehicle actually reaches the stop bar, vehicles can be prevented from idling at the intersection during low volume conditions.
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