The Folly of Double-Left Turn Lanes

There is a troubling, counterproductive “solution” that continues to be employed for addressing congested intersections – even in communities that are otherwise progressively promoting transportation choice. The “solution” is to add a second left turn lane to an existing left-turn lane when there is a perception that the number of motorists waiting in the single left-turn lane has grown too large.

Conventional traffic engineering claims that creating a double-left turn lane at an intersection is an double left turn lane intersection boulder“improvement” that will reduce greenhouse gas emissions by reducing congestion. And that a double left turn does not conflict with the transportation plan objective of promoting pedestrian trips.

On the contrary, I believe that double-left turn lanes will INCREASE emissions and will REDUCE pedestrian trips.

Double left-turn lanes cause serious problems for scale and safety for bicyclists and pedestrians, but have been shown to be counterproductive even if we are just looking at car capacity at an intersection. Adding a second left turn lane suffers significantly from diminishing returns. That a double-left turn does NOT double the left turn capacity – largely because by significantly increasing the crosswalk distance, the walk cycle must be so long that intersection capacity/efficiency (for cars) is dramatically reduced.

Cities across the nation are facing severe transportation funding shortfalls, yet at the same time, they are often building expensive and counterproductive double-left turn lanes.

Why? Probably because of the absurdity that transportation capital improvement dollars are in a separate silo than maintenance dollars, and that the former dollars are mostly paid by federal and state grants.  Of course, double-left turn lanes also destroy human scale and obliterates the ability to create a sense of place, but those are much more difficult arguments to make.

A colleague of mine adds that double-left turn lanes are an abomination. He adds that “they are a sign of failure: failure to provide enough street connectivity, so that when drivers do come to an intersection, it is gigantic, so it can accommodate all the left turns that had not been allowed prior to that point. Many trips on extra wide arterials are very short, and involve three left turns: one left turn onto the arterial and one left turn off the arterial: there trips could and should be made on connected local streets.”

How can a city claim it is short on transportation funding when it is building such counterproductive facilities? Double-left turn lanes…

  • Increase per capita car travel and reduce bike/pedestrian/transit trips.
  • Increase GHG emissions and fuel consumption.
  • Induce new car trips that were formerly discouraged (via the “triple convergence”).
  • Promote sprawling, dispersed development.
  • Discourage residential and smaller, locally-owned retail.

Cities need to draw a line in the sand: Place a moratorium on intersection double-left turn lanes and eventually removal of such configurations. Double-lefts are too big for the human habitat. They create a car-only atmosphere.

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Filed under Bicycling, Sprawl, Suburbia, Transportation

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