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.
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.