California's Coordinated Ramp Metering and Variable Speed Advisory Study Found a 14.56 Percent Decrease in Daily Average Number of Accidents.
Researchers Evaluated Combined Coordinated Ramp Metering and Variable Speed Advisory Strategies Field Tested on 17 On-Ramps Along a Freeway Corridor in California.
California
Field Test of Combined Coordinated Ramp Metering and Variable Speed Advisory for Freeway Traffic Control
Summary Information
Coordinated Ramp Metering (CRM) and Variable Speed Advisory (VSA) usually go hand in hand in function for freeway Active Traffic Management (ATM) in the sense that ramp metering controls the traffic flow into the freeway, while VSA affects driver behavior on the mainline. This study documented the field test of combined CRM and VSA initiatives along the State Route 99 (SR 99) northbound corridor from Stockton Boulevard to 12th Avenue near the crossing with US-50 in California. The field test lasted about one and a half years from May 2021 to November 2022, and involved 17 on-ramps and ten VSA signs along the study corridor, followed by a before-and-after analysis that compared a “before” scenario during 2015-2016 (no CRM nor VSA in operation) and the “after” scenario (both CRM and VSA in operation) during 2022-2023, using historical performance measurement system data, independent from field test data.
METHODOLOGY
This study combined the CRM and VSA algorithms, software, and hardware systems into one algorithm. The researchers investigated the dynamic interactions of the functionalities of CRM and VSA for real‐world freeway traffic using the SR 99 corridor data. The CRM algorithm used a simplified version of optimal control, called Model Predictive Control (MPC) with an objective function that is a tradeoff of the Total Travel Time (TTT) and Total Travel Distance (TTD) for the entire corridor. For the VSA algorithm, the researchers adaptively determined the VSA from the bottleneck to its upstream, by successively using proportional control, based on downstream flow, measured occupancy (or estimated density), and estimated distance mean speed.
FINDINGS
- Results revealed a 14.56 percent decrease in daily average number of accidents.
- This study also found that Vehicle Miles Travelled (VMT) increased by 4.5 percent.
- The efficiency (VMT/VHT ratio, where VHT is the Vehicle Hours Travelled), increased by 8.4 percent.
