Interstate 5 in Washington state to have variable speed limit

| Friday, April 11, 2008

In an effort to ease congestion around Seattle a six-mile stretch of Interstate 5 will switch to a variable speed-limit system in 2009. Instead of a single posted speed limit, electronic signs will adjust the speed limit throughout the day during peak traffic times.

One reason for the change is that Seattle is going to replace the Alaska Way Viaduct, which presently carries about 100,000 vehicles a day. When the Viaduct is shut down in 2010, much of that traffic is going to pour onto the already congested I-5.

So, on a six-mile stretch of northbound I-5, the state DOT plans to install a system of overhead speed limit signs and pavement detectors that will automatically change the speed limit to account for congestion.

The speed limit could go from 60 to 40 in the blink of an eye. And Washington DOT’s Ted Trepanier says drivers will learn quickly to take the variable speed limits seriously.

The Seattle Times reported that the system called speed harmonization is in use on some European highways. The idea is that drivers near the Seattle-Tacoma Airport, for instance, might be held to 30 or 40 mph, instead of going the standard 60 mph –  only to meet stop-and-go traffic downtown.

The system is expected to cost about $25 million.

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