Wavetronixby Don Leavitt, Wavetronix LLC, USA
SafeArrival tracks the speed and range of individual vehicles and extends the green light for any vehicle detected in the dilemma zone.
Intersection safety is a critical issue for communities around the world. In the USA, it has been identified as a national priority: according to the National Highway Transportation Safety Administration, traffic collisions at or near signalized intersections occur every 30 seconds, or roughly 3,400 per day, eight of which are typically fatal[1].
US transportation officials are paying particular attention to the problems associated with signalized intersections. This includes red light running, which results in approximately 15 percent of crashes at signalized intersections in the US, and accounts for roughly 33 percent of the nation's intersection-related fatalities. It also encompasses sudden acceleration through the intersection, resulting in right-angle collisions, which account for 28 percent of intersection crashes. Abrupt stopping, meanwhile, results in the more common rear-end collision, representing about 42 percent of intersection accidents. Each year, right-angle and rear-end collisions account for two-thirds of signal-related intersection accidents in the USA and according to the US FHWA have a societal cost of more than US$30 billion attached to them.
By providing comprehensive protection to the dilemma zone—and by tailoring that protection to the speed and range of each vehicle in the zone—this intersection system is setting new standards for efficiency and safety.
To combat these problems and improve the safety of their intersections, traffic engineers often feel forced to sacrifice intersection efficiency. Many intersection systems effectively increase safety but disrupt the normal flow of traffic, and vice versa. Wavetronix has introduced SafeArrival technology specifically to address these problems, while bridging the gap between safety and efficiency. Available only on the Wavetronix SmartSensor Advance, SafeArrival technology works with patented Digital Wave Radar to provide what the company claims is the most effective intersection detection device in the industry.
Wavetronix's SmartSensor Advance.
The US FHWA says that driving “near or through intersections is one of the most complex conditions drivers will encounter.” Many factors contribute to this complexity and while some are environmental and beyond a driver's direct control—such as weather or changes in light—most are created by the drivers themselves. These include traveling significantly faster or slower than the posted speed limit, the driver's reactions to other drivers and pedestrians, and the rate at which drivers misjudge the duration of signals.
Perhaps the most notable factor contributing to the complex conditions at or near intersections is the existence of the driver's decision dilemma zone. This dilemma zone is an area generally 2.5 to 5.5 seconds away from the intersection stop bar in which, when the light turns yellow, a driver must decide whether to stop suddenly, and risk a rear-end collision with the vehicle behind him, or accelerate through the intersection and risk running the red light and potentially experiencing a right-angle accident.
In an age of increasingly aggressive driving behavior, a defensive driver in the dilemma zone must weigh up these risks, and in a split-second, determine which option is the least dangerous alternative.
Although the actual location of this dilemma zone is theoretical and varies from intersection to intersection, the zone itself is very real, and the decision it presents to drivers can be very dangerous, especially on high-speed approaches. This dilemma zone, combined with the driver behaviors already listed, leads directly to the problems of red light running and abrupt stopping, and greatly increases the likelihood that a right-angle or rear-end crash will occur.
As traffic officials cannot reliably predict what an individual driver will do when faced with this decision, their only course of action is to somehow manipulate the signals themselves to give drivers enough time to avoid the decision dilemma zone altogether.
Theoretically, the yellow light should last long enough to allow a vehicle traveling at the design speed of the approach to comfortably stop on red if they cannot clear the intersection. But there is no perfect way to program the yellow light without knowing ahead of time what individual drivers will do: if the yellow light lasts too long, it can disrupt traffic flows and negatively impact the intersection's efficiency. And if the yellow light doesn't last long enough, it can create a physical dilemma zone in which drivers cannot safely stop or go. The best solution is a system that extends the green light based on individual vehicles: an ideal device to achieve this green light extension is the SmartSensor Advance with SafeArrival technology.
SmartSensor Advance's real-time view of the roadway.
Green light extension systems are an effective way of combating red light running and reducing the risk of both rear-end and right-angle collesions. They protect the dilemma zone by monitoring behicles as they appraoch the intersection, and holding the green light for detected behicles. A 10-year study of green light extension systems at high-speed, signalized intersections found that they reduced right-angle collisions by 31 percent, rear-end collisions by 75 percent and overall incidents of red light running by 65 percent[2]. However, despite the obvious benefits, a 2004 ITS deployment study found that less than half of the agencies surveyed are using green extension systems for dilemma zone protection.
There are a variety of reasons why agencies might choose not to implement green light extension. Firstly, traditional detection devices for dilemma zone protection are often difficult to install and impossible to maintain. Second, successful green extension systems need accurate and reliable detection of vehicles throughout the entire dilemma zone.
Brad Giles, an engineer at Wavetronix, reckons the SmartSensor Advance directly addresses both of these problems, while providing extremely effective dilemma zone protection: “SmartSensor Advance is easy to install, requires little or no maintenance, and is proved to be the most consistently accurate radar traffic sensor,” Giles says.
SmartSensor Advance bridges the gap between intersection safety and efficiency by tailoring dilemma zone protection to the speed of each vehicle.
SmartSensor Advance installs above ground and can be mounted either overhead or at the roadside. As part of the SmartSensor family, it uses patented Digital Wave Radar to detect vehicles in its field of view; it also uses the traffic industry's only patented automatic configuration process to expedite installation and setup and to ensure the consistent accuracy of its detections.
Giles says Wavetronix's Digital Wave Radar, coupled with the company's breakthrough SafeArrival technology, provides the accurate advance detection of vehicles essential for effective green light extensions. “This is the first technology that addresses both the protection of the dilemma zone and the red light problems that endanger drivers at the intersection,” says Giles.
SafeArrival technology works by simultaneously detecting the speed and range of every vehicle as they approach an intersection; it uses that data to then determine the estimated time of arrival (ETA) of each vehicle at the intersection's stop bar. If the ETA of any of the vehicles is determined to be unsafe, the technology will signal the traffic controller to extend the green light, giving those vehicles ample time to pass safely through the intersection.
“The power of SafeArrival comes from the fact that it protects each individual vehicle based on the speed it is actually traveling, and not based on the design speed of the approach,” explains Giles. “This way, SafeArrival can customize management of the intersection based on the vehicles and conditions that exist at any given moment.”
According to Giles, the bespoke protection improves safety by eliminating the decision dilemma zone. By extending the green light, SafeArrival greatly reduces the potential for drivers to run the red light and decrease the risk of right-angle collisions. By ensuring that all endangered vehicles make it safely through the intersection, it also eliminates the need to stop abruptly and risk a rear-end collision. Additionally, SafeArrival keeps traffic moving until a safe gap in the flow is detected: it avoids unnecessary delays for vehicles that are not in the dilemma zone, so the efficiency of the intersection increases. SafeArrival also effectively manages congestion by detecting queues, platoons, saturation and other traffic conditions will in advance of the stop bar., thereby providing more efficient control of isolated or coordinated intersections.
This bespoke protection is possible as a result of the combination of Digital Wave Radar and SafeArrival technology, which continuously monitors each vehicle it detects at a rate of every 5ms. Digital Wave Radar extends the reach of advance detection to 500ft, eliminating the need for trenching and resulting in a broad, 400ft view of the roadway. It also provides SafeArrival with consistently high resolution, even at far ranges: since radar is range-based technology, a 20ft zone resolves as easily as 500ft as it does at 50ft. Together, these technologies offer complete protection of the entire dilemma zone, improving the safety and efficiency of the entire intersection based on real-time events and conditions.
The SafeArrival technology found on SmartSensor Advance can see all approaching lanes, so the entire dilemma zone is effectively protected.
Of course, SafeArrival is not the first technology used for green extension systems, but the Wavetronix approach has several advantages that make it a more effective solution to the red light problem. Loop-based point detection has been the most widely used technology for dilemma zone protection and green extension and, according to Giles, it has been adequately effective but has several drawbacks.
First, loops must be embedded in the road: an installation process that is difficult, time-consuming and costly. In comparison, SmartSensor Advance's above-ground installation can be achieved in a fraction of the time and cost required for loops, without disrupting traffic. Once installed, loops are difficult to maintain, a critical problem when you consider the technology's failure rate of 20 percent. “Failures at single points are common with loops and they can cause the entire system to fail,” Giles says. SmartSensor Advance requires little or no maintenance and operates accurately in all weather conditions.
Secondly, loop-based point detection is based on the design speed of the approach, so is most effective when vehicles travel at the right speed limit. Drivers often deviate from posted limits and loop-based systems can't adequately protect those vehicles traveling over or under the designated speed. On the other hand, SafeArrival's advance detections are based on the actual speeds of each vehicle on the approach, so each vehicle is correctly protected, even if they aren't traveling at the speed limit.
Third, loop-based systems can only detect vehicles at single points in the approach, so each detection is just a snapshot of what's occurring at one point in the dilemma zone. This limits the amount of information that loop-based point detectors can collect about any given vehicle and also minimizes the amount of protection that can be offered. With SafeArrival, SmartSensor Advance can see the entire dilemma zone at once. As the arrival time, speed and range of multiple vehicles approaching the stop bar are all continuously monitored, SmartSensor Advance can alert the traffic controller instantaneously without requiring any delay.
SmartSensor Advance application in green light extension systems.
The power of SafeArrival is not limited to just green extension systems. Giles says it is also an effective detection strategy for red light extension systems. “If the light has changed to red and SmartSensor Advance detects a vehicle that is likely to run the red light, it can tell the controller to hold all the lights red until that vehicle passes,” Giles says. “That driver would still be cited for running the red light, but the extended red period would help prevent them from killing or injuring other motorists.”
In addition, SafeArrival can be used in other applications. With its capability to accurately determine the speed and range of vehicles in a broad 400ft view, the technology is effective for wrong-way and over-speed detection systems. It can also be configured to call and then extend the green and red intervals of a phase dynamically. One application that is gaining in popularity, and for which SmartSensor Advance and its SafeArrival technology are particularly well-suited, is the advance warning system.
Advanced warning systems use a warning flasher or sign to notify drivers that the light is changing will in advance of the intersection. Typically, the traffic controller will trigger the warning sign a few seconds ahead of the light change: a 'Prepare to Stop' message appears on the sign, alerting drivers before the light changes to yellow. These systems are especially effective at intersections with sight restrictions, in rural settings, where drivers typically travel at high speeds for extended periods without encountering intersections, and at high-speed, urban intersections or intersections prone to problems caused by heavy vehicles.
According to Giles, SmartSensor Advance is an ideal solution for this type of installation. Armed with its bespoke protection and Digital Wave Radar's broad detection range, it can detect a safe gap in traffic several seconds upstream from the dilemma zone. “It can tell the traffic controller to initiate the warning sign and then, when the detected gap passes through the dilemma zone, it can tell the controller to change the light to yellow,” Giles says.
The real power of SmartSensor Advance is its capability to effectively protect each individual vehicle detected in the dilemma zone. The combination of SafeArrival and Digital Wave Radar allows traffic engineers to improve the safety of their intersections without sacrificing their efficiency.
[1] Traffic Safety Facts, 2004
[2] Zegeer, C.V., and Deen, R.C., “Green Extension Systems at High-Speed Intersections,” ITE Journal, November 1978