Keeping drivers safe at the intersection is an ongoing challenge. It requires an understanding of what drivers will do when given a yellow light: will they stop suddenly and risk a rear-end collision with the vehicle behind them; or will they try to “beat the light” and risk running the red light or a right-angle collision in the intersection? Both options have risks, but running the red light is particularly dangerous. According to Federal Highway Administration estimates, nearly 900 fatalities occur each year in the United States from red light running alone.
This moment of uncertainty is known as the dilemma zone. Helping drivers avoid the dilemma zone (also called dilemma zone protection) by detecting them in advance of the intersection has proven to be an effective way to reduce red light running and increase intersection safety, but traditional methods fall short by either under-protecting vehicles that need it, or overprotecting vehicles that don’t, often at the expense of efficiency.
“SmartSensor Advance solves many of these problems by tracking a vehicle’s estimated time of arrival at the stop bar to provide the exact amount of dilemma zone protection required,” says Brad Giles, the Wavetronix product engineer responsible for Advance. “Advance adjusts dilemma zone protection as vehicles speed up or slow down, and it only provides protection at each vehicle’s true dilemma zone, benefitting both safety and efficiency.”
Dynamic ETA tracking is a relatively new concept in intersection control, and many traffic engineers question how effective it is. Now, two third-party studies to evaluate the benefits of dynamic ETA tracking have provided clear evidence that SmartSensor Advance significantly increases intersection safety by reducing the potential for red light running violations, without interfering with the efficient movement of traffic.
Traditional advance detection systems depend on inductive loops placed in the roadway at a fixed location some distance away from the stop bar. “The loop provides vehicle location and speed data, which are used by a controller to make phase-change decisions,” Giles says. “The location of the loop is often determined by the posted speed limit of the approach and corresponds to the start of the dilemma zone that would exist at that speed.”
Loops can be very accurate and have been the standard for vehicle detection for many years. But dilemma zone protection based on fixed-loop detections poses three problems. First, because loops detect at a fixed point, they exclude many vehicles from protection, decreasing the safety of the intersection. A vehicle’s true dilemma zone varies significantly based on its speed; traffic engineers will usually use 15th and 85th percentile speeds as the ranges to protect, but this leaves a large percentage of vehicles without full dilemma zone protection.
Second, loops detect vehicles only once and cannot detect speed. In some rare cases, two loops are used to create a speed trap, but these systems detect speed only once and cannot take into account changes in speed that invariably occur. “Drivers almost inevitably accelerate or decelerate as they approach intersections, and this variation in speed causes the dilemma zone for individual vehicles to also change,” says Giles. “Marking vehicle speed only once at a fixed location leads to inaccurate guesses as to where vehicles need protection.”
Third, loop-based dilemma zone protection utilizes much larger protection zones in an attempt to address both 15th and 85th percentile speed ranges, but this means that many vehicles are overprotected, or protected when they don’t need to be, and this leads to greater inefficiency. “Effective dilemma zones are significantly different depending on vehicle speed,” Giles says. “So in loop-based scenarios, vehicles end up getting protected before or after their actual dilemma zone.”
Inadequate protection does nothing to reduce red light running or the risk of collision. In contrast, SmartSensor Advance uses radar to dynamically track each vehicle’s ETA throughout its detection range, beginning 600 feet in advance of the intersection; if vehicles are detected in a dilemma zone, the sensor places a call to the controller to extend the green light until these vehicles safely pass. “Advance constantly updates each vehicle’s ETA as its speed changes,” Giles explains. “Over- and under-protection is avoided because vehicles are only protected within their true dilemma zones, so intersections end up being both safer and more efficient.”
Results of the research showed a 58% reduction in red light running per hour. Researchers also saw a 63% reduction per 1,000 vehicles.
Recent studies have demonstrated the effectiveness of SmartSensor Advance in improving intersection safety, particularly in relation to the occurrence of red light running. In the first study, conducted by Bradley University in Illinois, researchers evaluated an intersection for 36 hours to determine the “before and after” effects of using SmartSensor Advance.
For the first 18 hours, the intersection was monitored without Advance; the intersection used a conventional dilemma zone protection system using loops for detection; SmartSensor Advance was then activated, and the intersection was monitored for another 18 hours.
The study noted that the Advance system using dynamic ETA tracking significantly reduced the opportunity for max-out of the green light time, which resulted in fewer opportunities for vehicles to run the red light. Results of the research showed a 58 percent reduction in red light running per hour, from 2.80 in the “before” period, to 1.22 after Advance was implemented. Researchers also saw a 63 percent reduction in red light running per 1,000 vehicles, from 6.03 before to 2.24 after.
“The safety improvements in this test were attained on top of an existing fixed dilemma zone protection system,” Giles notes. “In situations where no dilemma zone protection system is in place, it is possible that additional gains in safety can be achieved.”
At one intersection, researchers reported a 38% reduction in crashes. At another intersection, the study reported a 100% reduction.
In the second study, the Ohio Department of Transportation’s Region 5 office in Jacksontown, Ohio, compared a traditional, loop-based dilemma zone protection system to a system utilizing Advance’s dynamic ETA tracking to determine whether or not the dynamic system made any significant difference in terms of intersection safety. The multi-year study monitored conditions at three separate intersections: prior to July 2012, these intersections used loops and “Prepare to Stop When Flashing” signs; these were then replaced with SmartSensor Advance.
At one intersection, researchers reported a 38 percent reduction in crashes, from 39 in 2011 to 24 in 2012. At another intersection, the study reported a 100 percent reduction in crashes, from 5 in 2011 to zero in 2012. Researchers noted that crash frequency was diminished by reducing the incidents of yellow and red light running.
“There seems to be less confusion and people aren’t trying to hurry up before the lights change,” said Lt. Craig Cvetan, the Lancaster Post commander for the Ohio Highway Patrol.
These studies demonstrate the power of dynamic ETA tracking over traditional advance detection systems. SmartSensor Advance significantly reduces over- or under-protected zones and inaccurate detections that lead to intersection inefficiencies, by constantly monitoring vehicles and adjusting true dilemma zones to match a vehicle’s changing speeds. Unlike traditional systems, Advance does this, not once, but multiple times while a vehicle is detected in its range. The results from these studies prove that Advance effectively reduces red light running to create safer, more efficient intersections.
