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Atlanta turns to variable speed limit system to relieve highway congestion
Transportation planners in Atlanta, ranked 11th in North America for worst city traffic, are reaching into their bag of tricks for a way to control driving speeds based on real-time conditions.
The Georgia Department of Transportation will be installing new electronic variable speed limit signs along the northern half of Interstate 285. The speed limits will range between 45 mph and 65 mph, and they will fluctuate depending on traffic volume and weather conditions.
"We have been considering this legislation for about three years," said Georgia DOT Commissioner Keith Golden. "We want to give drivers a speed limit that they'll be more compliant to."
According The Atlanta Journal-Constitution, the idea is to reduce stop-and-go driving and discourage frequent lane changes.
The whole system will be a relative easy project to set up, according to Golden. Work on the installation of the electronic signs is set to begin next month, and the system should be up and running by next summer.
“You’re able to move more traffic through with the variable sign system," Golden said.
Atlanta ranked as 11th worst traffic city (PDF)
Georgia isn't alone in this way of thinking. Washington, Virginia, Wyoming and other states have implemented a variable speed limit system along their highways.
According to a study by the Wyoming Department of Transportation, DOT officials from those states contend the system is working. Pat Persson, a district engineer in Wyoming, said the public reaction in general has been "very positive about these variable speed limits."
Traffic seems to be a concern for a lot of states. This month, the Texas Department of Transportation approved an increase in the speed limit on Texas State Highway 130 to a record 85 mph. The reasoning is the same as for Georgia - to help move traffic more efficiently.
According to Golden, there isn't necessarily a time frame for success for the Georgia project; however, if it manages to help congestion, then expect other highways in the Atlanta area, such as I-85, to receive the system as well.
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Or a speed limit sign that says their is a minor fender bender up ahead please slow down and look.
Why 65? Few people drive that slowly. Make it at least 70. Ideally 80mph in good weather.
Can anyone explain how increasing the maximum speed limit (or lowering it) will reduce traffic congestion? I live in the greater LA area, and frequently find myself driving in terrible stop-and-go traffic there. When traffic is bad, your max speed is set by the car in front of you, and you don't get anywhere near the speed limit (65 over most of the region's interstate system). As far as I can gather, the problem is that when density of cars gets high enough then every time someone changes lanes, room has to be mad for them in their destination lane, slowing that lane down. If the originating lane was moving faster, they slow it down as well when matching the speed of the destination lane. When the traffic density is low, this can be absorbed by traffic flow with little disruption. However, in very dense traffic, the effect lingers – you can get compression waves propagating in the direction opposite to flow. Also, some people will try to respond to a slow-down by changing to a lane that seems faster, which can exacerbate the effect.
Am I missing something? How in the world would increasing the max speed help that? Lowering the max speed I suppose might make the flow smother (ie: less variation in traffic speed, but still a reduced mean flow rate) – but its hard to believe anyone will obey that.
Some cheap fix idea from bored city planners, with no real budget to work with. That's my take.
buoy u shur r a gud speller
Here in Portland its about lowering the speed just before certain problem areas — you know those places on the freeway where there's an interchange and another couple of exits, or a major on-ramp just a few hundred yard before a big off-ramp.
With this kind of system then at rush hour they can reduce the speed in these areas and prevent the type of sudden breaking that — during heavy traffic especially — can be the whole initial cause of a traffic jam. It sends "traffic shockwaves" so to speak, back through the line of traffic that can take hours to untangle.
I wish more people realized this. If they did there might not be as much of the selfish driving that say ... causes people to rush ahead in a lane that they know is ending, and then try and ram their way in when the lane ends, causing everyone to brake back up the line.
You've put almost as much thought into traffic as I have. Here are a few more points to consider. The way to get the most cars onto the highway is to turn it into a parking lot. That works both ways: more cars slower, fewer cars faster. And the relationship is non-linear. A few more cars can make things a lot slower. Or anything that causes cars to change lanes, as you mentioned, will slow things down as well.
The purpose of metering lights is to prevent too many cars from entering the highway at any given time, which allows the traffic on the highway to travel faster, which makes room for more cars, believe it or not. They help stabilize the traffic flow.
It would appear that the purpose of the variable speed limits is to accomplish the same thing, to smooth out the traffic so things can work better instead of having people rush forward where they can to then pile up at the end of the next compression wave you mentioned. This might help the compression waves to dissipate. You won't spend more total time on the highway, just less time at the worst congestion spots.
Obviously as you mentioned, raising the speed limit won't help at all once things are congested. But it might reduce the size of the congested areas, both in time and in space. Before congestion starts, a higher speed limit would allow vehicles to get to their destination (exit) more quickly, making room for more vehicles, and thus potentially delaying when congestion begins. It is also possible that in some situations a higher speed limit would allow vehicles that have finished passing though a congestion area can leave it more quickly which might shrink the size of the congestion area. This won't help in those cases where the edge of the congestion area is a significant off-ramp or the opening of an additional lane.
I can only hope that this is all being designed by experienced traffic engineers who are executing a lot of detailed traffic simulations to realistically demonstrate the effects of these changes in their cities. Far too often decisions like this are made by people who intend well, but who lack the understanding of what their proposals will actually do.
Greg,
The faster the people can go the more cars can pass through and people get faster from point A to point B. I am from germany and believe me, faster speeds, not allowing to pass on the right and more educated drivers will take care of ALL traffic jams in the USA. GUARANTEED! Germany uses this system for ages and it works.!
So rather than increasing availability to mass transportation (buses and trains), Atlanta and other cities want to raise the speed? This raises fuel consumption and this may ultimately raise fuel prices as demand would increase. Not sure if this is really a good long term solution.
We are at the limits of what roadways can do in moving people in densely populated areas.
High speed rail is the only answer to this congestion.
Study after study shows that we either start moving to integrated rail systems or gridlock will star negatively impacting our economy.
This won't work If I was passing through one of these variable speed zones, I'd be travelling 1/2 a mile an hour faster than everyone around me. But seriously sounds like a great opportunity for a high speed mass transit system. Don't build speed traps. Build real infrastructure.
Sounds mostly like a great excuse to increase the city's income from speed-related citations.
Yes that is the other part of the brilliant idea from the city planners office: More glut in the courts generating revenue for the city. Those judges are so slow moving in their long flowing robes, Let's make them do the hustle!
This is not new, btw. Bellevue, WA (right next to seattle) has variable speed limits on SR-520. Electronic signs describe even tell you about congestion problems, forewarn you about draw bridge openings (there is a bridge on SR 520 connecting Bellevue and Seattle because Lake Washington is in between) and suggest alternate routes.
I believe that reducing the speed limit during peak times allows people to change lanes more quickly. Think about it: if you were in a slow moving lane and trying to switch into a fast moving lane, it would take you a while to find an opening, slowing down the people behind you. If that other lane were moving a bit slower, you could switch in faster, freeing up the path for people behind you. That's just my interpretation.
Officer: Do you know why I stopped you?
Driver: No? I was doing the speed limit.
Officer: Actually, no you weren't. The speed limit dropped 20 mph just as you went by. Sorry but I have to give you a ticket for going 20 mph over the speed limit.
Driver: This isn't fair.
Officer: The judge might be lenient. Drive safely please.
I think I saw Sheriff Roscoe P. Coltrane do this on the Dukes of Hazzard.
Too many people. It's only going to get worse.
Traffic can be moderated by installing IP controled throttling systems in cars. if there is a congestion ahead the computer would limit a persons car as to how fast they can go. This would keep the wave of backing up traffic from happening. This technology is very easy to implement on any car. It is nothing more than a govenor. If there is a huge traffic jam this would stop speeders and others who disobay the traffic laws the ability to speed by, by pass or add to the problem. Just a thought.
Its the poor drivers who cause the traffic jams. The person who cant merge onto the freeway without cutting people off or slamming on their breaks when no one "lets them in"? The person who will never in a million years move out of the fast lane to allow faster traffic to pass by? As soon as you get the opportunity to pass this ridiculous driver on the right....you just get stuck behind the next one in the fast lane....its never ending stupidity on our roads!!!! Take away their licenses therefore making them use public transportation. It improves traffic in two ways....gets the incapable drivers off the road while also reducing the overall numbers!
Traffic congestion in Wyoming? Nobody lives in Wyoming.