Dilemma Zone (DZ) is a section of the roadway where drivers are prone to making inappropriate decisions, resulting in a safety risk with the yellow indication at high speed intersections. DZ’s location and length are actually different for each approaching vehicle, depending on the vehicle’s speed and distance to the intersection at the yellow indication. The existing DZ protection systems cannot provide vehicle-specific warnings; therefore, their effectiveness has been limited. A DZ protection system with onboard warning devices is introduced in this paper, which provides protection to each individual vehicle by monitoring the vehicle speed and stopping sight distance in real time. This onboard warning system has the capacity to provide vehicle-specific warning, which addresses the shortcomings of existing systems. This DZ protection with onboard warning system consists of a control computer, onboard equipment (OBE), roadside equipment (RSE, including Signal Phase and Timing, SPaT device), and a mobile device such as a tablet or a smartphone. The onboard equipment calculates the DZ location and the time to the stop bar based on the vehicle’s current speed and global positioning system (GPS) coordinates. The calculation is performed 10 times per second when the vehicle is within the transmission range of the roadside equipment. If the system predicts that the vehicle is about to get into the DZ, the visual and audio warnings will be issued to the driver via the mobile device. This onboard warning system has been evaluated on the test bed of Transportation Operations Laboratory (TOL) at Turner-Fairbank Highway Research Center, and the test data was analyzed to evaluate the effectiveness of the DZ protection with onboard warning system. The performance of the onboard warning system is compared with that of an infrastructure-based warning system, developed earlier under this project. The result shows that drivers tend to start braking earlier or at a farther distance to the intersection with the onboard warning, which indicates less hesitation to follow the warning instruction (Be Prepared to Stop When Flashing), and a better trust on the warning information.
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