The Safe Green Passage application using the Multipath Signal Phase and Timing (SPaT) broadcast message to inform drivers revealed that as travel demand increased, the benefits of the application decreased.
Simulation modulating speed to increase the odds of encountering a green light.
Made Public Date


United States

Multipath Signal Phase and Timing Broadcast Project

Summary Information

The Multipath Signal Phase and Timing (SPAT) Broadcast Project demonstrated the Safe Green Passage traffic signal application, which provides speed guidance to an approaching driver so that a vehicle may safely pass through the green phase of an upcoming traffic signal. This is accomplished by the signal system’s ability to send SPAT information to approaching vehicles even when they are several miles or multiple signals away. The simulation performed to evaluate performance of the Safe Green Passage application used three intersections along Grand River in Novi, Michigan (Drake Road, Halstead Road, and an M-5 exit ramp). This site was chosen because accurate signal timing information was available. Field testing of the application occurred at two signalized intersections in Oakland County, Michigan.


In a field demonstration performed on August 8, 2011, SPaT information was sent from the signal controller by cellular and Dedicated Short Range Communications (DSRC) radios and was received by the vehicle’s on-board device. The desired speed is calculated based on current location, vehicle speed, and scheduled signal phase changes. If needed, a recommended change in speed is made to the driver by the device’s display.

To evaluate the potential performance of the approach, a simulation was performed using intersections that represented the typical environment and were already included in the Vehicle to Infrastructure (V2I) testbed. The four metrics measured included Travel Time, Total Delay, stopped delay, and number of stops.


The field test was successful demonstrating how the traffic signals and vehicles would function in an environment using the SPaT broadcast message and the Safe Green Passage application. The evaluation performed used a traffic simulation model to replicate a system of 25,000 vehicles produced some of the following varied results:
  • As travel demand increased, travel time increased as did stop delays.
  • As travel demand increased, travel times changed from improving by 2 percent to declining by 2 percent.
  • As travel demand increased, stop delays increased.
  • As travel demand increased, the number of stops increased by 30 percent.
  • Increases in market penetration rate showed decreases in effectiveness.
  • Using SPaT produces a 15–25 percent increase in travel time.
  • As distance between intersection increases, the overall system performance improves.
  • Reductions can be significant when close to intersection (up to previous intersection).
  • The system impact is small on a large network if only a few intersections are equipped.
This particular study found, through simulation, that as travel demand increased, the benefits of the Safe Green Passage application decreased. While on the surface, the Safe Green Passage application seems to have potential to assist the driver, in-depth analysis of the details required to implement the application reveal many weaknesses that may inhibit its usefulness. It was decided after this demonstration that further development of the Safe Green Passage application would not be pursued at this time.