Monday, March 21, 2016

District 7 Installs Its FIRST Three-Lane Staggered Ramp Meter

A member of the electrical team adjusts a signal lens.
Ramp meters may be one of the most underappreciated, misunderstood congestion-busting strategies in the traffic operations toolbox. Motorists sometimes grumble about them, believing that having to stop at an on-ramp meter lengthens their commute. However, studies have conclusively demonstrated that meters increase freeway speeds, decrease travel times and reduce accidents.

Ramp meters work so well that Caltrans has installed more than 1,100 of them in District 7—but only one is a three-lane staggered ramp meter. Since February 4, this new ramp meter strategy has been safely and efficiently shepherding vehicles onto southbound I-405 from the Valley Vista Boulevard on-ramp.

The first three-lane staggered ramp meter is installed.
What exactly is a three-lane staggered ramp meter? First, some ramp metering basics. Ramp meters are installed on freeway entrance ramps and connectors to control the flow of vehicles entering the freeway. They’re designed to decrease congestion and improve average vehicle speed by controlling vehicle flow, since vehicles entering at short intervals are less likely to cause bottlenecks. Additionally, meters enhance safety by reducing rear-end and sideswipe collisions.

The new three-lane staggered meter at Valley Vista Boulevard discharges vehicles separately for each metered lane using split timing. Each of the three signals has a unique cycle such that each motorist gets a green light at a different time. The software that makes this possible also allows the Ramp Metering Branch to program different discharge rates for each lane. For example, the left lane might discharge a car every six seconds, while the middle and right lanes discharge cars every eight seconds.

The sluggish Valley Vista Boulevard on-ramp to southbound I-405 was a prime candidate for the district’s first three-lane staggered meter. During peak hours, traffic was backing up on both local streets and the freeway. The existing meter, which had been discharging three vehicles at the same time, simply wasn’t getting the job done.

So why not discharge two cars per lane per green instead of one, also known as platoon metering? It wouldn’t have worked well at this location. Platoon metering would cause traffic to back up to the ramp meter signal from the merging point on the freeway, causing gridlock and increased travel delay.

The inside of the ramp meter cabinet.
Getting the new three-vehicle staggered meter system up and running at the Valley Vista ramp was no easy task. It required two days of splicing wires, pulling cable, updating the ramp meter cabinet and programming the controller to accommodate the new software for staggered metering. Additionally, the signal lenses had to be adjusted so they’re not visible from neighboring lanes, an operation that required a lot of painstaking trial and error to get the positioning just right.

Once the new hardware and software was installed and the lens signals were perfect, ramp metering engineers closely observed traffic behavior on local streets and the freeway over a period of several weeks to determine the impact of the new staggered metering. Based on their careful observations, they tweaked the signal cycles to achieve the best possible timing for the location.

Six weeks later, delays are shorter and traffic is flowing more smoothly on local streets near the on-ramp and on the freeway. The underappreciated ramp meter has once again proved itself to be a congestion buster to be reckoned with.

More info about Caltrans ramp metering is here.