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The Case Against One-Way Streets
Jan 31, 2013
By Eric Jaffe
Read More: http://www.theatlanticcities.com/commute/2013/01/case-against-one-way-streets/4549/
PDF Study: http://www.uctc.net/access/41/access41-2way.pdf
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Cities have long been home to one-way streets because transportation engineers believe they move cars better than two-way streets do. That's largely the case because one-way streets eliminate tough left turns through oncoming traffic. Any way around conflicting lefts, on two-way streets, creates congestion: left-turn lanes take up space, and guarded signals take up time. Vikash Gayah, a civil engineer at Penn State University, isn't so sure about that conventional wisdom. In addition to the aforementioned reasons to convert one-way streets, Gayah believes congestion will improve as well.
- The typical metric of traffic is vehicle flow — which amounts, more or less, to standing on the corner and counting how many cars go by. Flow is high on one-way streets because there's little reason for cars to slow down. But flow doesn't take into account the fact that traveling through one-way street systems often means taking a circuitous route, which adds distance to every trip. "You can move more vehicles through a roadway, but if they have to travel a longer distance, in the end, you have actually fewer people being able to get to their destination and get off the road," says Gayah. Instead, Gayah prefers a metric called "trip-serving capacity," which considers both the flow and the extra travel distance created by a street system.
- So cities looking to improve trip capacity in downtown areas have some options. Smaller cities, with shorter average trip lengths, should be able to reduce congestion by converting one-way streets into two-way streets (with a couple options for left turns). Larger cities, with longer trip distances, should consider a shift to two-way systems that ban left turns entirely. And that's just focusing on traffic. The benefits of switching to two-way streets mentioned at the top of this post only sweeten the deal.
- Even if a city isn't willing to convert its one-way streets quite yet, Gayah's trip-capacity work shows the wisdom of banning left turns at existing two-way intersections. (That's something U.P.S., which doesn't let its drivers turn left, has known for years — and New York appears to be learning this lesson too.) Since such a change carries a very low implementation cost, that's a great place for cities to start. "I think if a city was willing to try that they would see some significant benefits in the long run," he says.
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For very short trips, two-way streets perform about as good or better than one-way streets, whose flow can't compensate for the additional distance. Over longer distances one-way streets start to perform better, but never quite up to the trip capacity of two-way streets with banned left turns (the dotted red line). The beauty of this type of system is that it combines the flow of a one-way street with the directness of a two-way street.
Jan 31, 2013
By Eric Jaffe
Read More: http://www.theatlanticcities.com/commute/2013/01/case-against-one-way-streets/4549/
PDF Study: http://www.uctc.net/access/41/access41-2way.pdf
.....
Cities have long been home to one-way streets because transportation engineers believe they move cars better than two-way streets do. That's largely the case because one-way streets eliminate tough left turns through oncoming traffic. Any way around conflicting lefts, on two-way streets, creates congestion: left-turn lanes take up space, and guarded signals take up time. Vikash Gayah, a civil engineer at Penn State University, isn't so sure about that conventional wisdom. In addition to the aforementioned reasons to convert one-way streets, Gayah believes congestion will improve as well.
- The typical metric of traffic is vehicle flow — which amounts, more or less, to standing on the corner and counting how many cars go by. Flow is high on one-way streets because there's little reason for cars to slow down. But flow doesn't take into account the fact that traveling through one-way street systems often means taking a circuitous route, which adds distance to every trip. "You can move more vehicles through a roadway, but if they have to travel a longer distance, in the end, you have actually fewer people being able to get to their destination and get off the road," says Gayah. Instead, Gayah prefers a metric called "trip-serving capacity," which considers both the flow and the extra travel distance created by a street system.
- So cities looking to improve trip capacity in downtown areas have some options. Smaller cities, with shorter average trip lengths, should be able to reduce congestion by converting one-way streets into two-way streets (with a couple options for left turns). Larger cities, with longer trip distances, should consider a shift to two-way systems that ban left turns entirely. And that's just focusing on traffic. The benefits of switching to two-way streets mentioned at the top of this post only sweeten the deal.
- Even if a city isn't willing to convert its one-way streets quite yet, Gayah's trip-capacity work shows the wisdom of banning left turns at existing two-way intersections. (That's something U.P.S., which doesn't let its drivers turn left, has known for years — and New York appears to be learning this lesson too.) Since such a change carries a very low implementation cost, that's a great place for cities to start. "I think if a city was willing to try that they would see some significant benefits in the long run," he says.
.....
For very short trips, two-way streets perform about as good or better than one-way streets, whose flow can't compensate for the additional distance. Over longer distances one-way streets start to perform better, but never quite up to the trip capacity of two-way streets with banned left turns (the dotted red line). The beauty of this type of system is that it combines the flow of a one-way street with the directness of a two-way street.
