Syncing traffic signals no easy task

  • Sun Mar 25th, 2012 9:24pm
  • News

By Bill Sheets Herald Writer

Ralph Gilcreest of Mukilteo writes: I’m bothered by unsynchronized traffic lights. Just think of what good could be done if the cities and counties could time their traffic lights to coordinate with the speed limit. There would be less idling time, which would save fuel and commute time. There would be less incentive to speed, which should improve safety. With the chaos of random signal changes, speeders do get places faster.

Synchronizing the traffic lights only needs to be sold to a few government officials. Getting people to merge safely, or use their turn signals, needs to be sold to tens of thousands — and many of them probably don’t read The Herald! The mathematics of timing the traffic signals, at least on the main roads, can be worked out by a junior high school math class.

Jim Bloodgood, traffic engineer and traffic operations manager for Snohomish County, responds: Synchronizing or coordinating traffic signals is not as simple as one might think.

It is, of course, easy if one is dealing with an isolated, one-way street. In that case it is a simple application of speed and distance. It becomes more complicated when dealing with two-way operations and more complex yet when dealing with a network of both north-south and east-west arterials — then throw in a few diagonal roads just for fun.

Snohomish County cannot speak for other agencies but traffic signals on our main corridors operate in coordinated plans.

Operating in a plan means that a signal runs at a set cycle length (the time it takes to serve all movements), including “splits” (the time allotted for each direction) and “offsets” (the time between when one signal turns yellow and the next one turns yellow).

We also work with the state Department of Transportation to coordinate our signals with freeway ramp signals and other signals on state highways. However, our signals do not operate in coordinated plans 24-7. There are times, such as just after the morning commute, where traffic is light and we let the signals operate in “free” mode to reduce side-street delay. Free means a signal responds to traffic volume up to certain maximum intervals with no specific pattern.

After about 8 to 9 p.m. we let the signals operate in free mode. Our isolated standalone intersections also operate in free mode because the greater the distance between signalized intersections, the less the ability to coordinate them.

Four-way stop

Alex Link of Lynnwood writes: I have a question regarding the stop sign at the intersection on Meadow Road and 137th Street SW in the Martha Lake area. I drive this road daily.

I understand having the stop sign for drivers going south and west, because you could not see a car that was about to drive down the hill there when reaching Meadow Road from 137th.

However, why can’t it be a two-way stop instead of a four-way stop? Why does the northbound driver on Meadow have to stop when there is enough visibility to see if a car is coming up the hill? When the temperatures are below freezing and ice forms on the roadway I am not going to come to a complete stop at that intersection and possibly become stuck on a hill.

Is the county able to switch it to a two-way stop?

Owen Carter, engineer for Snohomish County, responds: Drivers expect uniformity in traffic control devices. Making this a two-way stop would create an unusual situation that could lead to driver confusion and result in a less-safe intersection.

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