SPaT data were collected at three separate intersections within the VCC, including a 4-way intersection in a dense area, a T-type intersection near a small shopping center, a major arterial, and a nearby interstate corridor in a suburban area over rolling hills. Two data delivery paths were explored: 1) Cellular (SPaT messages broadcasted from RSU to be received on the Laptop via VCC Cloud); and 2) DSRC (SPaT messages broadcasted from RSU to be received on the OBU). Data collection occurred over the course of one day at each site and was broken up into separate morning and afternoon sessions, wherever possible, to cover different levels of traffic demand. Data was collected over 60- to 90-minute runs.  Latency data logged at various locations and GPS locations logged during the tests were used to plot a spatial observation over longitudinal distance. Analysis on the distributions of latency for DSRC and cellular data (Table 1) shows that DSRC has a shorter range but very low latency (less than 2 ms), whereas cellular has a longer range but higher latency.

Table 1. Summary of average latencies and ranges using DSRC and cellular for all three sites.

Source: Feasibility Study and Assessment of Communications Approaches for Real-time Traffic Signal Applications, December 1, 2020, ITS JPO

Feasibility test of four applications using DSRC and cellular suggests that receiving SPaT data over cellular might enhance the performance for applications such as Glidepath and traffic optimization for signalized corridors (TOSCo), as the delay induced by cellular may be negated by the message being received over a wider distance (Table). However, applications such as transit signal priority (TSP) and Red-light violation warning (RLVW), which require low latency, may not be supported by the cellular network.

Table 2. Feasibility of applications using DSRC and cellular.

Source: FHWA.
*Cellular may be acceptable for TSP at speeds ≤ 50 miles per hour.