A Speed Nudge?

Speed limit signs tell the driver how fast they can legally drive. What if they actually told them something more useful — namely how fast to drive so that one is assured of not having to stop at the next light?

The following press release came to me recently:

Information Display Company, a leading developer and manufacturer of radar speed sign technology today announced the launch of TrafficFlow Manager, a driver alert display that works with traffic signal timing to alleviate traffic congestion. When mounted along a route with timed traffic signals, the display informs drivers that the lights are synchronized and lets them know the proper speed they must maintain in order to avoid having to stop for a red light.

The benefits of light synchronization are obvious; for example:

A report issued by the U.S. Department of Transportation showed that a traffic light synchronization program in Texas reduced delays by 24.6 percent and fuel consumption by 14.2 percent. A similar program in Austin Texas saved commuters 2.3 million hours of their time and 1.2 million gallons of fuel usage.

There are obvious limitations — for one, the systems are more expensive than conventional lights (and not sure where that money is coming from in current municipal budgets); for another, when competing traffic demand is high, synchronization schemes often break down.

Another question is whether an unbroken string of green, in urban areas for example, encourages speeding. Which is why I was intrigued by the idea of this “functional” speed limit sign. As I’ve said before, I’m constantly amazed by the inefficiency of drivers in New York, accelerating from red light to red light, often beyond the speed limit itself. But the question begs: If, upon getting a green, if drivers knew that driving 28 mph would get them to the next light when it was going to change, and that to go faster would simply cause them to have to come to another stop, would they actually drive at that speed?

This strikes me as in the spirit of a Nudge — there’s nothing (except that pesky old law) standing in your way of making any speed choice you like, but you are given clear information on what the best choice is. Still, questions loom — what if someone joins the street from a side-street, and has no idea of where they are in the synchronization scheme, and may drive faster for fear of the light turning red.

This entry was posted on Monday, June 22nd, 2009 at 9:04 am and is filed under Traffic Engineering, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

23 Responses to “A Speed Nudge?”

  1. Frank Says:

    Or, the signs now give the motorists even more “reason” to be agitated by those not going the synchronized speed – such as bicyclists and pedestrians.

  2. chrismealy Says:

    The street in my picture is my hometown, Longview, Washington. They’ve had a “go 28 and you’ll make all the lights” sign since at least 1992, when I noticed it, and maybe earlier. It works really well. You really can make all the lights across town.

  3. Charlie Says:

    Sometimes this is already true but signs don’t tell you as such. For example, on a main road where I live, I discovered if I go just under 30 mph I hit all the lights green. There is another town that I’ve traveled through where signs indicate that signals are timed for 25 mph. They indeed are! It’s a lot more pleasant not having to stop and go at each light. However, if there were unsignalized pedestrian crossings or many bicyclists on the roadway, I would be concerned that motorists trying to time the lights would get frustrated.

  4. chrismealy Says:

    BTW, in Longview the road with the 28 mph green wave is the route that connects I-5 with Route 30 to the Oregon coast. It’s the kind of long-block wide road where people would normally try to go 35-40 mph, so in its case ambling along at 28 mph is good traffic calming.

    Street View:,-122.941099&spn=0.004023,0.013787&t=k&z=17&layer=c&cbll=46.127225,-122.941138&panoid=Gdz1vuYDbtSY77i5kzSqLw&cbp=12,20.91,,0,-3.4

  5. Vincent Clement Says:

    The answer to your question is yes. I recall driving through downtown Hamilton, Ontario, that with a constant speed you hit most if not all of the green lights. The majority of people were driving at this ‘magic’ speed.

  6. Elaine Says:

    Chris, I know that spot too! It does seem to calm a street that I think would otherwise be at LEAST 35mph. It’s nice to be able to just cruise along w/out hitting any red lights.

    In my town, I know of at least two places where the lights are timed for just below the speed limit, without any signs for it. I think most people still do the speed up & stop thing, unfortunately. Downtown Olympia seems particularly bad in this regard.

  7. James Edward Pearce Says:

    On the other hand, I frequently see motorists past me (a cyclist) and even other cars when the light ahead is clearly red.

  8. aaron Says:

    It’s about f’ing time.

  9. aaron Says:

    Now we just need to get rid of speed limits.

  10. aaron Says:

    This is why I think ITS isn’t ready for prime time. First we need to get rid of speed limits have posted speeds that coordinate with the traffic lights. Only then will ITS be worthwhile.

  11. aaron Says:

    The obvious answer to the street entry is have these systems is the driver goes whatever speed until they see a sign, then simply adjust.

    The other option is allow the the signs to broadcast a signal that GPS systems or other device can pick up.

  12. aaron Says:

    28mph does look like way too low of a speed for the road in that steet view. Could be an attempt to manufacture congestion.

    Wouldn’t be at all suprised to find no safety benefit and that fuel efficiency went up partially due to reduced stops, but would be even better with higher cruising speeds. And that some of the efficiency gain was actually due to traffic volume going down, driving efficiency down somewhere else.

  13. aaron Says:

    The company’s primary focus does seem to be reducing speeds, not improving actual safety or efficiency. The improved efficiency of less stops is probably just a marketing tool to sell a congestion creation system.

  14. aaron Says:

    The effectiveness studies Information Display Company uses to sell it’s products (do not focus on efficiency, these systems are sold as safety products, and are not related to the speed/stop light coordination system), do not show any improvement in safety. They only evaluate effectiveness in reducing speeds (reducing fuel efficiency, time efficiency).

    The only evaluation that even mentions safety, suggests that it does not improve safety. This CHP Memorandum is from IDC effectiveness studies document (emphasis added):


    Date: February 2, 2007

    To: Humboldt Area

    Humboldt Area

    File No.: 125.11998


    In 2006, there was a significant reduction of collisions at three locations in the Humboldt Area. The reduction of collisions was associated with engineering changes made by the California Department of Transportation (Cal Trans). These changes include changing the roadway surface from conventional asphalt/concrete pavement to “opengrade” asphalt/concrete pavement and posting additional sings, some of which are radar activated signs that indicate the speed of approaching motorists. Two of these areas are located on US 101; the third area is located on SR-299.

    The north most area is located on US 101, between mile post markers 101 HUM 124.71 and 125.98; a stretch of highway frequently referred to as “the curves just south of the by-pass, north of the old fish hatchery.” In 2004, there were nine reported collisions in this area. In 2005, there were eight reported collisions in this area. Cal Trans completed a safety project, which included a change to “Open Grade” roadway safety surface, and the addition of radar activated signs in December 2005. In 2006, there were 5 reported collisions in this area.

    The second area on US 101 is located between milepost markers 101 HUM 109.42 and 112.53; an area commonly called “the Big Lagoon Curves.” In 2004, there were 12 reported collisions in this area. In 2005, there were also 12 reported collisions in this area. Cal Trans completed a safety project in this area that included radar activated signs and a change to “open grade” roadway surface in December of 2005. In 2006, there were 5 reported collisions in this area.

    The third area is located on SR-299 on the east slope of Lord Ellis Summit, between milepost markers 299 HUM 19.05 and 20.67. Though the curves are not as tight as those on US 101 are, the roadway grade is steep. Both in 2004 and 2005 there were 12 reported collisions in this area. Though no comprehensive safety project was conducted in this area, Cal Trans conducted a number of incidental improvements and repairs. This area does not have radar activated signs. In 2006 there were three reported collisions in this area.

  15. gpsman Says:

    “If, upon getting a green, if drivers knew that driving 28 mph would get them to the next light when it was going to change, and that to go faster would simply cause them to have to come to another stop, would they actually drive at that speed?”

    Probably not, unless the timing incorporates the average rate of acceleration from a stop to 28mph, probably around 2-3 seconds.

    Third St. in downtown Cincinnati (SL 25) lights are timed at about 18-20 mph, including a rather slow rate of acceleration. Leave first at the light, drive 18-20 mph and traffic will pass, change lanes to your front and fill up that space with 6-10 vehicles within a block (catching the red, of course) leading the light-timer to have to stop, and then, of course, to miss a downstream green they are attempting to time.

    Practically nobody is willing to spare their time to compare the feeling/s of velocity against actual results. Apparently, progress is very often measured by only vehicles passed.

  16. Kenneth Todd Says:

    The idea that better traffic signal synchronization will reduce travel time and relieve congestion is very attractive and easily sold to the public. Drivers always hope to hit green at the next light.
    That’s why the experts extol the wonders of synchronized traffic signals but forget to give us the bad news. These systems function only with moderate traffic volumes and usually in one direction only. Those who travel in the opposite direction and on the side streets pay for it with longer delays. That’s why they measure the savings of time and fuel only on the road where the traffic signals are synchronized.
    But when there are more vehicles than fit through the green, some are left behind to wait for the next green and the next bunch has to stop till they are cleared. And when a driver turns into the coordinated flow from a side street,
    he gets to the next light when it shows red, so that the next bunch again gets stopped by a stationary vehicle. Only when we are lucky do we get faster to the next bottleneck, where we have to wait that much longer.

  17. Allan Says:

    What about this: you turn onto the street and just make a yellow. you can then speed up to like 40 and get progressively earlier in the green. Assuming this is a grid and you are going diagonally, you can turn right at the beginning of the green and then gun it to make the next set of lights. you have defeated the speeds :)

  18. Ed Says:

    @aaron, there is a huge safety benefit to a pedestrian of 28mph traffic versus 35-40mph traffic.

  19. Mark Young Says:

    Catching a ‘green wave’ has safety and environmental benefits (not just fuel efficiency, but also local pollutants from all the stopping), so – notwithstanding all the traffic modelling issues discussed here – this one has my support.

    These kinds of ideas can be taken further, too – I was at a standards meeting recently where they were discussing new in-car telematics systems that give you timers on when the lights are going to change (you might have seen similar things actually on the traffic lights in some countries in Asia). This was mooted as a way of ‘nudging’ drivers to slow down early if the lights were about to change red – but I bet them any money that if drivers had that sort of info, they’d speed up rather than slow down.

  20. aaron Says:

    Ed, I need numbers and data. Pedestrian and vehicle volume and travel distances.

    The road pictured doesn’t look like one that would have increased ped incididents with higher speeds.

    I think it’s mostly BS. 28 is still pretty damn fast for a pedestrian impact and I doubt it decreases the rate per VMT and PMT. Probably increases it.

  21. aaron Says:

    (PMT, I meant Pedestrian Mile Traveled, but Passanger Miles Traveled should also be looked, rather than just VMT. Of couse, payload value miles traveled should be considered too.)

  22. aaron Says:

    That’s another problem; rate of acceleration.

    Engineers assume a slow rate because they can’t tell people how bad slow acceleration is, but policy makers could be working on getting acceleration rates up. That would greatly improve speed and light timing, fuel efficiency, drive times, and possibly safety.

  23. bikeolounger Says:

    Yep, people speed often in areas where the lights are synchronized. I see it daily on my commute. Often, though, if it’s a grid of one-way streets, there are enough lanes that the real hot-dogs stay in one lane, allowing those who “get” synchronized lights to pass merrily along.

    It would be cool to see the sync speed come down to about 20, allowing bicyclists to get further per light cycle, especially if there is one of those pesky bike lanes from which we can taunt the motorists (by simply being there–not actively taunting). We are trying to get this to happen locally, if not to 20, then to 25 or 28 from 35.

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