Smarter Highways 102: Variable Speed Limits
Tuesday, September 21, 2010 by guest blogger Annie Johnson
It’s about what’s ahead
Just like the symbols, the speeds displayed on the overhead electronic signs warn you about what’s happening on the road ahead of you. Congestion on the highway ebbs and flows and the variable speed limit signs reflect that fluctuation. It’s just like a driver speeding up and slowing down to adjust to the traffic flow. The only difference is that the Smarter Highways signs let you know in advance that the speed change is coming.
Speed limits reflect the conditions
The electronic speed limits displayed on the signs reflect travel conditions not necessarily how fast or slow traffic is moving in the freeway lanes. If traffic is stopped, the signs will display the system’s lowest speed, 40 mph. The signs won’t show a lower speed limit, even if traffic is moving slower.
Think of the signs just like a regular static speed limit sign. It’s the maximum speed you should travel, however as any driver knows you might have to drive slower than the posted speed limit due to traffic or weather. Also, just like a static sign the speed limits are enforceable and you could be ticketed by law enforcement for exceeding the posted speed limit.
Helping drivers travel safer and smarter
Ideally, approaching traffic will slow down and pass through the problem area at a slower but more consistent speed reducing stop and go traffic. By reducing stop and go traffic we’re also reducing the probability of an accident by giving drivers more time to react to changing road conditions. This helps drivers avoid the need to brake sharply as they approach congestion.
So how do you come up with that speed?
In urban areas traffic sensors along the roadway collect vehicle speeds, congestion information and traffic flow rates. This information is continuously relayed back to our Traffic Management Center in Shoreline and analyzed by computers. When circumstances that would benefit from lowered speed limits—like congestion—are identified, the computer reduces speeds incrementally to gradually reduce the approaching flow of traffic to the congested area. This is different than the electronic speed limit signs you see on I-90 over Snoqualmie Pass and on US 2 over Stevens Pass. These signs are manually operated by our South Central and North Central region Traffic Management Centers to reflect the current road and weather conditions on the passes.
So there you go. That’s Smarter Highways 102. Any questions?
18 comments:
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If the signs are blank, is the speed limit 60 mph? What if the previous signs said 40 mph?
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When there are no incidents on the road ahead, the signs are blank and the speed limit is the standard 60 mph. Traffic conditions are constantly changing so it is possible that the previous set of signs posted a reduced speed, such as 40 mph. Don't worry about that - the sign in front of gives you the latest information as they update every minute.
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I've noticed that the speed signs flicker from time to time, and when they do that, they sometimes change speed, but sometimes don't. When they flicker but don't change, does that mean that the system just decided whether or not to change the speed, but ended up leaving it alone? (And, do they have to flicker when they don't change? I find it distracting.)
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@Anonymous: Yes the flicker means that the system was getting new speed limit information and the speed didn't change. We do continue to monitor the sytem and make adjustments. The flicker is one thing are are attempting to address. Thanks for the feedback!
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Patty ( I am guessing it is Michaud) says "don't worry".
Guess she is going to pay my fine?
Gimmeafrigginbreak, sayeth the Geezer.
How about the I-90 variable limit signs, that are yellow on black. Last I checked, regulatory signs were black on white background in this state, but that could have been in my youth, just a year or two ago.
The Geezer -
@The Geezer: There isn't a requirement that the variable speed limit signs be black and white. You can check out the Washington Administrative Code (WAC) 468-95-045 for all the speed limit sign requirements.
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So how long will it take and how many tickets will these signs have to generate to pay for themselves? I avoid I-5 through Seattle most of the time anyway, so I don't think I'm going to have to deal with this nonsense. But really, anyone who thinks this will improve that situation should get off his/her computer and take a drive through the affected area. I'm sure you'll find that these signs are simply a cruel joke and a way to "enhance" revenue, at taxpayer expense, of course.
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Hi Annie J,
FYI and some traffic engineering for you:
I think the geezer is correct if you read the WAC referenced in the reply. The WAC does not change the regulatory sign colors required in the MUTCD, pg 33 Table 2A-5. The only colors allowed, per the MUTCD, on regulatory changeable message signs are black and white. Only in special cases may purple be used. There is no other choice for regulatory sign colors on a changeable sign. If yellow is used it is a warning only, not regulatory. -
@WSDOT: I would be curious to hear your response to the comments about MUTCD. I also have a inquiry about the minimum speeds posted being 40. When coming up on a congested area I often will see 40 MPH signs but this does not give me an indication if traffic is going smoothly at 30 MPH or if it's stop and go. Wouldn't there be a benefit to slow traffic down even further? It also seems silly to have adaptive signs showing the speed limit as 40 when you are parked under one of the signs.
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What is the benefit of slowing traffic at all? Congestion starts from people driving slow. Police should enforce RCW 46.61.100 specifically subsection (4):"It is a traffic infraction to drive continuously in the left lane of a multilane roadway when it impedes the flow of other traffic." These "Smarter Highways" are more worthless than SR520's right side HOV lanes.
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I've been seeing the 'test' message for a while now. When Does WSDOT plan on having these signs on I-90 activated? Just curious.
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So what was the cost of purchasing and installing these signs?
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Where can we find objective data that shows the massive investment in smart highways infrastructure is actually making a positive impact on Washington (specifically the Puget Sound Area) traffic alleviation? On SR520 I've seen absolutely zero improvement in congestion or commute times. It's more of a joke to sit in stop and go traffic and see the signs saying the speed limit is 40mph. Please tell me these signs are actually producing positive results!
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@Anonymous: We need one to two years of data for a statistical analysis because traffic data can vary dramatically from month to month and year to year. The more data we have to compare the better. It also takes time compile and analyze the data but we hope to be able to share some of our initial findings regarding northbound I-5 Smarter Highways this winter.
Emergency responders, maintenance crews and our contractors tell us they feel safer when working under closed or blocked lanes with Smarter Highways in place. Drivers are more aware of closed or blocked lanes because they are getting out of the lane sooner and giving workers more room to safely do their work. -
@Patty: I've found the blocked lane notice to be a great opportunity to STAY in that lane as long as possible and cut in at the last second. It saves more time than an HOV lane! Glad to hear the project is benefiting the vast majority and not specifically commuters. That's a great basis for funds allocation. Clearly SR520 is a different traffic situation than I-5 so data from I-5 won't tell us what to expect from SR520 data. Don't get me started on the SR520 plan that does nothing to remedy the I-5 southbound ship canal bridge fiasco or the SR520 to Mercer street "wedge" - in both directions.
Another poster asked the total cost of the Smart Highways project. Can you tell us or direct us to a place to find it? Thank you. -
Curious why there's still no reply to the question about the project cost.
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@Anonymous: Smarter Highways consists of two projects: northbound I-5, which cost $23.8 million and I-90 and SR 520 which cost $42 million. These costs include both design and construction.
For future reference you can always find project costs on the project's website.
If anyone else has further questions feel free to email me directly at johnsan@wsdot.wa.gov. -
Thank you.
