Rethinking the One-Way Street
Two-way streets are good for the city; engineers say they are bad for drivers. But are they?
In the mid-20th century, many American cities converted two-way streets into one-ways. Many of these conversions were a response to the suburbanization of the middle class, who increasingly relied on cars to commute to central city jobs. Streets redesigned as one-ways made it easier to drive in and out from peripheral suburbs. Absent the friction created by oncoming traffic, they allowed for higher speeds over greater distances with fewer stops, making automobility more convenient.
Prioritizing city streets for fast traffic to pour motorists into and out of urban centers, however, severely compromised other transportation goals, such as safe travel, active trips, and clean air. A growing body of research argues that shifting city street design toward accommodating automobile-oriented suburban expansion has led to economic disinvestment in the urban core and more unpleasant — and often dangerous — streets for pedestrians and cyclists.
Accordingly, in recent years, more urban planners have begun arguing against one-way streets. Research suggests that converting streets back to two-way traffic can make cities safer, fairer, and more economically robust. Just as one-way streets make traffic faster, two-way streets can slow it down — improving safety, accommodating multimodal travel, increasing livability and property values, and helping customers more easily reach businesses. Two-way street networks create safer and more pleasant environments for those who walk, bike, use micromobility, or take transit.
Just as one-way streets make traffic faster, two-way streets can slow it down — improving safety, accommodating multimodal travel, increasing livability and property values, and helping customers more easily reach businesses.
Recent studies have found that converting one-way streets back to two-way has succeeded in many cities, often at a lower cost than anticipated. However, these studies also acknowledge that the original argument for one-way streets remains the most prominent argument against switching back to two-way streets today: One-way streets let cars move faster. Put simply, traffic engineers often argue that two-way streets will inconvenience drivers by slowing them down.
On the surface, this argument seems incontrovertible. If one-way streets let cars go faster, they must be better for drivers. But speed is only one component of travel time. The other component is distance, and one-way streets could force drivers to travel farther. This extra distance could eat into any gains allowed by higher speed. Given this relationship between speed and distance, there is no guarantee that one-way streets are always better for drivers. Our research asks: To what extent do one-way streets force drivers to travel farther?
Exploring the Efficiency of One-Way vs. Two-Way Streets
Our study compared the real-world street network in San Francisco, which includes one-way streets (an “as-is” network), to an imagined alternative where all the city’s one-way streets had been converted to two-way (a “to-be” network). We hypothesized that one-way streets increase the distance traveled because they often force drivers to circle around one-way blocks to get to their destinations. In other words, while one-way streets may offer drivers fewer stops-and-starts and higher speeds, two-way streets could offer drivers more direct routes and shorter trips. In tandem, these effects help determine trip length, travel time, fuel consumption, and ultimately vehicular emissions.
To test this hypothesis, we constructed models of the “as-is” and “to-be” networks, then simulated over a million daily driving trips on each to demonstrate how street conversions could impact distances traveled. We simulated two variations of driving trips. First, we used the California Household Travel Survey to gather real-world commute origins and destinations throughout the city. We complemented this approach by also using randomly selected origins and destinations across the city. The first approach offered us a glimpse of real driving patterns, and the second helped us evenly characterize the entire street network, since real-world driving tends to concentrate on certain streets.
The Efficiency of Two-Way Networks
In both cases, we find that efficiency — in terms of distance traveled to complete a trip — improves when the network comprises only two-way streets. All else equal, two-way street networks allow significantly shorter average travel distances. Intracity trips are approximately 1.7% longer on San Francisco’s existing mix of one-way streets than they could be if all streets were two-way. The average real-world commute trip is two blocks longer on the “as-is” network and the average randomized trip is three blocks longer.
While those extra couple of blocks per trip may seem trivial, they start to add up when you consider that millions of such trips happen every day. By our calculation, one-way streets are responsible for 27 million additional kilometers of vehicle travel annually, just for trips within San Francisco. Traveling even a couple of extra blocks in downtown San Francisco at rush hour often means several minutes of inching through clogged intersections. One-way networks may promise greater travel speeds, but we find they also result in greater travel distances.
The cost of “surplus” travel on one-way street networks may increase over time. Today, cities are grappling with the impacts of new mobility like ride-hailing, and with the prospect of autonomous vehicles and robotaxis that could continually cruise around on the streets. Such innovations hold the potential to generate even more miles traveled on our city streets. In such future scenarios, it becomes even more important to consider trip distance when designing street networks. Network capacity is finite, but two-way street networks can help cities manage increases in total travel distances.
One-way streets also have an inherent inefficiency, which leads to longer driving distances: They prevent drivers from taking the most direct route to their destinations.
Two-Way Street Conversions in Practice
Since the mid-20th century, planners have commonly assumed that one-way streets offer more efficient traffic flow. One-way networks can offer faster driving speeds, eliminate oncoming traffic, and avert the need for left turn lanes. However, our research shows that one-way streets also have an inherent inefficiency, which leads to longer driving distances: They prevent drivers from taking the most direct route to their destinations.
Two-way conversions can advance multiple urban planning and societal goals and must be considered when designing — or redesigning — street networks. This research offers practitioners an additional tool to broaden the dialogue around the advantages and trade-offs of two-way streets. One-way streets benefit drivers with increased speeds but penalize them with greater travel distances. Exactly which effect dominates — or if they merely cancel each other out — depends on traffic signalization, capacity, and congestion conditions that vary across streets and times of day. One-way streets benefit drivers; but in a full accounting, the benefit may be quite small. More importantly, two-way streets benefit everyone else. They can improve air quality, safety for cyclists and pedestrians, and promote patronage of local businesses. At the same time, context matters and street conversions are not a panacea: One-way streets can sometimes be an appropriate engineering solution; for example, on steep hillsides or locations where appropriate rights-of-way cannot be obtained.
Nevertheless, planners, engineers, and policymakers should think carefully about the full costs of one-way streets when they consider new engineering projects, particularly when considering a two-way conversion. They can use feasibility studies and tactical pilots to model the proposed redesign on a small scale and engage stakeholders before implementing it citywide. Given the benefits of two-way streets, conversions have started growing in popularity, and recent examples offer templates and case studies for others to follow. Cities like South Bend, Indiana, Louisville, Kentucky, Klamath Falls, Oregon, Austin, Texas, and Denver have recently converted one-way streets to two-way, and Toronto, Paris, and Atlanta have established plans for conversions. Our findings suggest that more cities should consider the ancillary benefits of two-way streets: They not only improve safety, livability, and local economic conditions but they can help reduce the total distance traveled on our streets.