Double left-turn lanes should not be used in Boulder. Conventional traffic engineering (including many statements I have read from City of Boulder staff) incorrectly claims that such intersection “improvements” will reduce greenhouse gas emissions by reducing congestion. Such an erroneous view also inaccurately believes a double-left turn does not conflict with the transportation plan objective of promoting bicycle and pedestrian trips.
In contrast, I believe that double-left turn lanes will increase emissions and will reduce pedestrian and bicyclist trips. Double-left turn lanes 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. A double-left turn does not double the left turn capacity. Why? By significantly increasing the crosswalk distance, the walk cycle must be so long that intersection capacity/efficiency (for cars) is dramatically reduced.
By creating excessive asphalt width for motorized vehicle travel, double-left turns (like dedicated right turn lanes or slip lanes) induce excessive motor vehicle turning speeds, and a dangerous level of motorist inattentiveness.
The enormous size and relatively high motor vehicle speeds induced by a double-left turn intersection creates dangerous and intimidating conditions for bicyclists and pedestrians, which substantially reduces the number of such trips and increases the number of traffic injuries and fatalities. Boulder’s newly adopted “Vision Zero” objective of reducing the number of annual traffic fatalities and serious injuries to zero will not be possible with such intersections, regardless of how much the City installs more safety lights, safety paint, safety signs, safety enforcement, and safety education. There is no win-win when it comes to cars. And Boulder continues to fail to understand that.
One of the preposterous aspects of this state of affairs is that Boulder previously planned to propose a tax on all property to raise revenue for road maintenance (there tends to be a severe budget shortfall for such funds), yet the City seems eager to build expensive and counterproductive double-left turn lanes. This is 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 or state grants. Of course, double-left turn lanes also destroy human scale and a sense of place. A double-left turn lane intersection will never feel like a place to hang out because it vastly exceeds human scale. These over-sized intersections are so hostile that they obligate property owners at each of the four corners of the intersection to pull back from the intersection with massive setbacks, large asphalt parking lots, and auto-oriented land uses that can tolerate such an unpleasant atmosphere (such as a gas station). This sort of deadening creates a area of apparent abandonment, and is the antithesis of what a city needs for health.
Ultimately the double-left turn intersection fails to induce nearby land uses that will generate tax revenues sufficient to make this part of the city self-supporting. It becomes an on-going financial liability that will forever drain substantial dollars from the city budget
Michael Ronkin, former bicycle/pedestrian coordinator for the State of Oregon, adds that double-left turn lanes are an abomination. He adds that “they are a sign of failure: failure to provide enough street connectivity. Without enough street connectivity, intersections must be gigantic so they can accommodate all the extra turns the motorist must make as they drive out of their way and circle back to their intended destination.
How can Boulder claim it is short on transport dollars when it is building such counterproductive and expensive 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 (triple convergence); promote sprawling, dispersed development; and discourage residential and smaller, locally-owned retail.
Boulder needs to draw a line in the sand: enact a moratorium on the construction of new intersection double-left turn lanes and eventually removing such configurations (unless they are necessary to avoid excessive congestion caused by a road diet).
Double-lefts are too big for the human habitat. They create a car-only atmosphere.