Usually, traffic adapts to road conditions, slowing as volumes increase. But with reversible highway lanes, roads can adjust to accommodate demand.

As Snohomish County commuters approach Seattle in the morning, the express lanes on Interstate 5 offer some a path out of the sea of brake lights. The lanes, which run from downtown Seattle to Northgate, then reverse to handle after-work congestion.

A tight schedule, with little flexibility, is posted for the lanes. Exceptions are made occasionally for special events, or for maintenance and construction work.

During the week they run southbound from 5 a.m. to 11 a.m., then at 11:15 the lanes shift and are open to northbound drivers until 11 p.m. when they close for the night.

On weekends the road heads south in the morning and then flips at 1:45 p.m.

As a frequent I-5 user, I wanted to know if the lanes could be more nimble? And perhaps switch direction to adapt to congestion from crashes or for higher-than-average, weeknight traffic volume that seems to be headed into Seattle after work Friday. So I posed the question to the state Department of Transporation spokesperson Joseph Calabro. (The lanes do go off schedule for major events and construction work.)

“Safety is the most important factor when deciding whether or not to reverse the I-5 express lanes in response to an incident. When dealing with reversible lanes, we have to be certain there is no traffic left in the lanes before opening them in the opposite direction. A lot of work goes into safely reversing the express lanes at an unscheduled time.

“We first call crews to the scene who have to be ready to fix any equipment issues that might arise. They also put up or remove the net at the Northgate entrance from southbound I-5. Once crews are in place, we begin the process of closing the lanes remotely from our Traffic Management Center. Then, our crews wait for traffic to empty out of the express lanes before following behind to confirm they are indeed empty. It’s at that point we begin opening the entrances on the opposite end.

“Before and during the process, we notify the traveling public and transportation partners including King County Metro (Transit), Community Transit and Seattle Department of Transportation. In all, the process can take up to an hour to accomplish when unscheduled, which is why we have to consider how long an incident will be blocking and how many lanes are blocked before making a decision to reverse the I-5 express lanes. By comparison, our scheduled reversals close the lanes for 10 to 15 minutes.”

WSDOT regularly conducts traffic analyses to confirm the express lanes are working on an effective schedule. Current average hourly traffic volumes indicate the express lanes are operating on an appropriate timetable, Calabro said.

So it seems that extra Friday evening traffic I fight through before my weekend can start is still less than outbound traffic heading north.

Got a question? Email me at streetsmarts@heraldnet.com or call 425-374-4165. Please include your name and city of residence.