Motorists should keep in mind that the HOV restriction on the SR 520 mainline west of Interstate 405, is for three or more persons. The purpose of ramp meters is to help regulate the flow of traffic onto a freeway corridor, reducing bottlenecks. Meters also reduce accidents at merge points, thus reducing accident-related congestion. Ramp meters on Interstate 5 north of downtown Seattle have decreased accidents by 39 percent, despite higher traffic volumes. Because access to the SR 520 mainline will now be regulated in most areas, traffic engineers hope to see mainline traffic moving more efficiently through the SR 520 corridor.
Ramp meters help reduce disruptions to mainline traffic to help the flow of traffic on the freeway.
They look like traffic signals and are located on freeway entrance ramps. Since ramp meters usually allow only one vehicle through each green light, the rate at which vehicles enter the freeway is controlled, usually every four to fifteen seconds. This creates a space between merging vehicles, reducing disruptions to mainline traffic and reducing accidents at merge points. Ramp meters are programmed to adjust meter rates automatically in response to real-time mainline and ramp traffic volumes. For a few weeks, operators will watch the meters through closed-circuit television cameras, and will override the computer system, if necessary, to manually operate meter cycles.