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Why Traffic Slows at Tunnel Approaches

Via ENR, an interesting piece on tunnels, shockwaves, and traffic psychology.


“If everybody goes the same speed and distance and nobody uses the brakes, you can through-put more vehicles,” Khattak said.

These tunnel-driving behaviors are so notorious that the proposal for the group interested in widening the HRBT suggests making the tunnel four lanes in each direction and the approaches only three lanes.

Scerbo noted, though, that regardless of how many lanes are added at the tunnels, many drivers will still brake.

Although some of these driver reactions can’t be controlled, the physical environment they’re reacting to can.

“We’ve tried to counter this whole perception that you’re driving into a hole in the water… so you don’t feel like you’re going into a different environment than you’re coming from,” said Dwayne Cook, regional operations manager for the Virginia Department of Transportation.

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This entry was posted on Wednesday, November 10th, 2010 at 11:40 am and is filed under Traffic Engineering, Traffic Psychology. 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.

14 Responses to “Why Traffic Slows at Tunnel Approaches”

  1. Stephanie Says:

    It’s dark and that’s scary on an instinctive level.

  2. Cap'n Transit Says:

    Allow me to point out that this is nowhere near as much of a problem with trains, where you’ve got one professional driver on a cleared block for hundreds of passengers.

  3. mulad Says:

    It’s an interesting problem, and I think we see shades of it whenever there’s a sunken freeway. The barrier of earth off to the side makes people more nervous as it gets closer to the roadway — A tunnel simply completes the enclosure and makes the space feel even more confining.

    There must be a few more things that could be tried — perhaps make the individual lanes wider or paint the ceiling blue to look more like sky.

    Unfortunately, I suspect that an effort to train people to just ignore the feeling of danger would ultimately backfire — if we take that feeling away, I’d worry about reducing the effectiveness of traffic-calming efforts elsewhere.

    I’m not familiar with these tunnels or the future project, but would additional buses help with the problem? If you can get people to take transit or carpool, that could reduce the number of vehicles that could trigger a backup.

  4. Betty Barcode Says:

    People slow down for all the right reasons, such as the recognition that an accident in a tunnel would be deadlier than one on an ordinary highway, where there are no walls to smash into. Also, the tunnel is presumably darker, and if you can’t see what’s ahead, it is appropriate to slow down. Mulad is right; why do we want to condition people out of good behavior?

  5. Glinda Says:

    They slow down to adjust to the darker light. A “pre tunnel” consisting of a series of arches that reduce direct sunlight could ameliorate the problem. Example: go to Google Maps and type
    51.230366,6.738746

  6. Scott Says:

    Yes, drivers should slow down when approaching a tunnel that they cannot see into.

    They should not be braking, however. Drivers should look ahead, notice they are approaching the tunnel, and decelerate gradually rather than hit the brakes.

    I think that better driver education would help. Too many people tailgate as well as jam on the brakes when they should simply decelerate more gradually.

  7. JJM P.E. Says:

    Given that the same thing happens at the Kosciusko Bridge near Albany, NY (a through-arch) I suspect that the sense of confinement has as much to do with it as going under a mountain or river. Drivers feel that they have less room for error, and slow down to compensate.

    It’s even been noted next to concrete barriers and other obstructions.

    I suggest maintaining shoulder width, or even making them wider, as well as tapering the entrance, rather than abruptly narrowing the roadside.

  8. fred_dot_u Says:

    JJM P.E. appears to have the answer for slowing down drivers in residential areas that have had the roadways expanded and “improved” to the point where speeds are excessive for any other uses in a safe manner.

    Let’s put up tunnel walls and tops and slow those cars down!

    yeah, not practical, but even a narrower roadway would probably help in that respect.

  9. Tony P Says:

    fred: On residential streets, the traffic calming tunnel is practical, cheap, pretty, reduces greenhouse gasses…and it works! It’s called a tree canopy.

  10. fred_dot_u Says:

    I agree, Tony P. So many benefits to such a roadway.

  11. Thomas Says:

    This is kind of to be expected. What I don’t understand is people who slow down to go through a green light.

  12. fred_dot_u Says:

    People who slow while entering an intersection might have had the unpleasant experience of encountering someone else who does not stop at red lights. Four decades ago, my driver’s ed instructor taught us to lift off the pedal and hover over the brake pedal when entering an intersection. Motorcycle safety classes suggest to have one’s hands on the brake and clutch levers at intersections.

    If I’m sitting at a red light that turns green, I do not arbitrarily enter the intersection. With a green light, I look left and right before continuing. It’s prevented one crash so far. That’s enough for me to continue the practice.

  13. Smellis Says:

    Glinda,
    I agree that Germans are design oriented and efficient, but i don’t think those “pre-tunnel” joists are meant to prepare drivers for entering a tunnel as much as they are there for structural purposes (to hold the wall that separates the 2 directions of travel).

  14. Eduardo Says:

    We face the same in Brazil, especially at the Rodoanel, a road ring around Sao Paulo city. Drivers will slow down or even brake at the entrance of a 2 km long tunnel, causing traffic to be jammed for up to 8 km (about 5 miles). The fear of the darkness and the unknown triggers the instinct of grabbing as much control as you can, so you would even swerve on the brakes if the tunnel was pitch dark, this I can undertand. In the other hand, modern tunnels are full of lights as bright as the sun, even in a tropical country like mine, so there is full vision ahead and yet the instinct prevails… I guess using more the reason and less the instinct could be bennefical for everyone… (as long as this does not reduce safety)

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