One-way or two-way streets more efficient? It depends on what you measure

By Chris Spahr

The debate over one-way versus two-way streets has been ongoing for more than half a century in American cities. Counter to prevailing engineering wisdom, a new study finds two-way streets may be more efficient, if one is measuring getting people to their destinations.

Many cities have recognized that two-way streets provide substantial benefits to downtown neighborhoods for a variety of reasons:

  • Two-way streets are better for local businesses that depend heavily on their visibility to passersby.
  • Two-way streets have been found to be safer than one-way streets. One-way streets correlate with higher speeds and decreased levels of driver attention. Pedestrians prefer crossing two-way streets since drivers tend to travel more slowly on them, and vehicular conflicts are more predictable.
  • Two-way streets are much less confusing for downtown visitors than one-way streets.  Visitors driving in a two-way grid network can easily approach their destination from any direction.

Various cities, including Dallas, Denver, Sacramento, Tampa, and Cedar Rapids have converted or are currently considering the conversion of one-way streets to two-way streets.

While there has been much agreement on the economic, safety, and livability benefits of two-way streets, traditionally traffic engineers and transportation planners have felt that one-way streets serve traffic more efficiently by allowing for a higher vehicle moving capacity. However, Vikash Gayah, of Penn State University, argues that the concept of “trip-serving capacity” is a better metric of network efficiency than vehicle moving capacity.

Gayah defines trip-serving capacity as the maximum rate at which people reach their destinations. While current research and conventional wisdom suggest that one-way street networks are more efficient than their two-way counterparts, this study shows that one-way networks are sometimes less efficient because they restrict the rate at which people reach their destinations.

Using the new metric of trip-serving capacity, Gayah compared one-way streets to several types of two-way streets (those with full left-turn lanes, those with left-turn pockets, and those that banned left turns). The study found that for short trip lengths (e.g., in small cities), the additional time spent traversing street grids associated with one-way networks created a lower trip-serving capacity than that of two-way networks. Over longer distances (e.g., in larger cities) one-way streets perform better but never exceed the trip capacity of two-way streets with banned left turns. In other words, the two-way network with banned left turns always has a higher trip-serving capacity even when trips are long.

Gayah encourages urban planners and traffic engineers to examine his trip-serving capacity concept when considering converting to two-way streets. “Since residents prefer two-way street networks for a variety of reasons, converting a one-way street network to a two-way operation can improve both efficiency and livability of cities.”

Chris Spahr is a Graduate Assistant with SSTI.