Bike lanes are commonly supposed to be a traffic calming device. Narrow lanes and narrow roads have slower traffic so adding a bike lane to a wide road should turn it into a narrow road, so goes the reasoning.

But what is it about a narrow road that encourages drivers to slow down?

There is increasing evidence, and a logical explanation, that exempt bike lanes as traffic calming devices.

Traffic calming is accomplished by the apparent angular speed of 3-D stationary objects as the motorist goes by. These objects include trees, parked cars, light poles, signposts, etc. The closer they are to the passing motorist, the greater their apparent angular speed as the motorist passes. A solid stripe on the road does not have this effect. It does not contribute to a sense of close by hazards.

Now a bicyclist in the bike lane is a nearby 3-D object but the bike lane strip actually tends to negate the calming proximity effect of the cyclist, in terms of inducing the motorist to proceed more slowly.

In spite of initial beliefs to the contrary, studies of actual traffic situations have shown that passing motorists are more likely to slow down and give more room when overtaking a cyclist on a roadway without a bike lane stripe, than on the same width roadway with a bike lane stripe. So, in terms of relative traffic calming value, the ranking probably goes like this:

  1. cyclists on wide outside lanes
  2. cyclists in a bike lane
  3. empty bike lane

The bike lane stripe gives the motorist a sense of confidence and assurance that the space is wide enough to proceed without conflict. Because of this effect, it has always seemed to me that bikelanes are actually pro-motoring facilities even though they are most always proposed by anti-motorists.

References:

http://www4.trb.org/trb/crp.nsf/All+Projects/NCHRP+15-18

(more when I re-locate them)