Ramp meters

Learn about ramp meters and how they work.

Image of a ramp meter signal on an on-ramp with displays dark (off).

Ramp meters are a specific type of traffic signal installed to control how quickly vehicles enter traffic flow on a freeway. Ramp meters are a type of freeway operation strategy designed to reduce collisions and decrease travel times. A typical ramp meter will allow one vehicle to enter the freeway on each green light, which creates a delay of anywhere from 4 to 15 seconds between vehicles. This delay helps reduce merging issues, which is usually the cause of most on-ramp related disruptions to freeway traffic.

How to use a ramp meter

Ramp meters use computer algorithms that take in large amounts of traffic data, which is then analyzed to determine the most effective rate of allowing more vehicles onto the freeway within certain maximum and minimum limits.

We use traffic data collection devices, such as inductive loops and side-fired radar, to collect traffic speeds and volumes on freeway lanes and ramps. The responsible Traffic Management Center gets this data. The computer system analyzes the data and automatically adjusts the ramp meter signal timing rate to maximize traffic flow on both the freeway and the ramps.

How ramp meters work

Ramp meters use computer algorithms that take in large amounts of traffic data, which is then analyzed to determine the most effective rate of allowing more vehicles onto the freeway within certain maximum and minimum limits. We use traffic data collection devices, such as inductive loops and side-fired radar, to collect traffic speeds and volumes on freeway lanes and ramps. The responsible Traffic Management Center gets this data. The computer system analyzes the data and automatically adjusts the ramp meter signal timing rate to maximize traffic flow on both the freeway and the ramps.

Benefits

When heavy traffic uses an on-ramp without a ramp meter, multiple vehicles try to merge at the same time. Drivers on the freeway now have to adjust to multiple vehicles, rather than just one at a time. When drivers try to accommodate multiple vehicles entering the freeway by slowing down, they slow everyone else behind them down, quickly growing into a backup and congestion. Freeway drivers may also slow down too much or too fast or be uncertain what to do, which could result in a collision.

When an on-ramp is metered, only one vehicle is trying to merge, which is more comfortable for drivers on the freeway to accommodate. It reduces how often and how much drivers slow down for the merging traffic—resulting in fewer disruptions to freeway traffic, including slowdowns and collisions. A short wait on the ramp helps keep average freeway speeds higher, helping reduce travel times overall.

Installations

Ramp meters are installed at on-ramps in most major metropolitan areas, where the freeways are frequently congested. Ramp meters are installed in the following metro areas and freeways:

  • Seattle/Tacoma/Everett: Interstates 5, 90, and 405; Highways 16, 18, 99, 167, 512, 518, 520, and 522
  • Olympia: Interstate 5
  • Vancouver: Interstates 5 and 205
  • Spokane: Interstate 90

Ramp meters typically operate during peak congestion times: 6 AM to 9 AM, and 3 PM to 7 PM. Meters may still be operated outside these hours, as their operation depends on freeway traffic speeds and volumes, and not on time of day.

Quick facts

  • Ramp meters generally reduce collisions system-wide by 30%
  • Ramp meters are a proven and cost-effective method of relieving traffic congestion
  • After ramp meters were installed on Interstate 405 in Renton, travel times were reduced by 3 to 16 minutes.

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