Planning Magazine

The Path to Safety: How Road Diets Can Save Lives

A comprehensive new study shows that narrowing lanes may curb fatalities, save money, promote walking and biking, and mitigate urban heat.

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Narrowing lanes offers opportunities to consider all users by adding bike lanes or other pedestrian-friendly amenities, according to a study from Johns Hopkins Bloomberg School of Public Health. Photo by The Image Party/

Crossing the street to get to Main Elementary School in Rome, Georgia, can be downright dangerous for kids. Brittany Griffin, a former senior planner of the northwest Georgia city and now an impassioned advocate for the community, has been trying for years to implement traffic calming measures or install a pedestrian crossing signal along a four-lane, 100-foot-wide stretch of Georgia State Route 293, just a stone's throw north of the school.

While there is a nearby speed camera that aims to slow drivers, there are no traffic lights or safe sections where students can cross when traffic guards are not on duty. To cross at an intersection with safety measures, they would need to walk about a half-mile west. The roadway has been in place since the 1960s, when urban renewal "ripped apart" the low-income Jewish and Black neighborhoods that make up this section of Rome, Griffin says.

She is fighting for the roadway to be narrowed, the curb to be extended, and the medians to be expanded and planted with trees to provide cover for the kids and other pedestrians.

The issue in Rome, Georgia is a snapshot of a problem vexing communities big and small across America. Increasingly, pedestrians and bicyclists are being killed on roadways. In 2022, more than 7,500 pedestrians were killed by drivers, according to a study from the Governors Highway Safety Association.

New research — A National Investigation on the Impacts of Lane Width on Traffic Safety, published by the Johns Hopkins Bloomberg School of Public Health in late 2023 — suggests that the solution may be narrowing road widths, sometimes referred to as a road diet.

While the American Association of State Highway and Transportation Officials (AASHTO) Green Book recommends lane widths between 10 and 12 feet, the study found that traffic lanes that were nine, 10, or 11 feet had a lower accident risk or frequency compared to 12-foot lanes. The study — which analyzed 1,117 streets in seven cities (Dallas, Denver, Miami, New York, Philadelphia, Salt Lake City, and Washington, D.C.) — also found that wider lanes with higher speed limits had significantly more crashes than narrower lanes.

Data from a Johns Hopkins study shows that road widths of 9 feet had significantly fewer crashes in the 30 to 35 mph speed limit range than those with wider lanes. Source: A National Investigation on the Impacts of Lane Width on Traffic Safety.

Source: A National Investigation on the Impacts of Lane Width on Traffic Safety

"Other benefits of lane width reduction are increasing roadway capacity, promoting walkability, and inclusive use of streets by all travel modes," the study authors write. Narrowing lane width also can reduce construction and maintenance costs for urban arterials and collectors, the authors add.

Finally, the report reads, "narrowing lane width would address challenging environmental issues by accommodating more users in less space, using less asphalt pavement, less land consumption and smaller impervious surface areas, and the consequent effects on the occurrence of urban heat islands in cities."

Data shows narrow lanes are safer

Shima Hamidi, PhD, an assistant professor of environmental health and engineering at Johns Hopkins Bloomberg School of Public Health and the director of the Center for Climate-Smart Transportation at Johns Hopkins University, served as the study's principal investigator along with planning professor Reid Ewing, PhD, who is the Distinguished Chair for Resilient Places at the University of Utah. Hamidi says the number of fatal pedestrian versus automobile collisions is "shocking."

"If you look at the fatality rate in the U.S. — 11.66 per 100,000 population as compared to 1.3 to 3.2 in European countries, almost 10 times higher — it's just worse for pedestrian and cyclists," says Hamidi. "[The year] 2020 marked the deadliest year for pedestrians in 40 years and is just increasing over time. There was a 40 percent increase just between 2010 to 2018 for pedestrian fatalities."

Hamidi says that streets should prioritize the most vulnerable users. And, if a street is safe for pedestrians and cyclists, she says, "then the street is safe for everyone — including drivers."

Narrower lanes also force drivers to pay more attention — a critical safety factor, says Chris Comeau, FAICP CTP, senior transportation planner at TranspoGroup.

When he was the senior transportation planner in Bellingham, Washington, the city received a federal grant to reduce the number of fatal and serious collisions on Alabama Street, a four-lane, two-and-a-half-mile road that connects several low-income neighborhoods. A road diet was implemented, taking it from four lanes to three, with a bike lane on the western third of the corridor, a center turn lane in the eastern third, and few changes to the middle section, which also carries a high-frequency bus route. The whole corridor also received ADA upgrades, sidewalk improvements, and six pedestrian High Intensity Activated Crosswalk (HAWK) signals at bicycle crossings.

Comeau conducted studies in the years that followed. He found that the number of collisions resulting in injuries was reduced by about 30 percent from before the changes, while travel times increased by just one minute.

A new study from Johns Hopkins Bloomberg School of Public Health offers suggestions for how planners can reduce traffic lane widths to improve pedestrian safety, such as considering the needs of all users, startng with 10-foot lane widths in slower speed areas, using context to determine appropriate speed, and creating a lane repurposing program to support additional bike lanes and expanded sidewalks. Source: A National Investigation on the Impacts of Lane Width on Traffic Safety.

Source: A National Investigation on the Impacts of Lane Width on Traffic Safety

Drew Pearson, AICP, senior planner at Wilson & Company, Inc., Engineers & Architects, is working on a similar project to bolster safety for all users of Southwest Boulevard, a five-lane, one-and-a-half-mile corridor in the heart of downtown Kansas City, Missouri. The project — when fully designed — may involve a lane reduction with pedestrian crossing areas at key locations, parking on both sides of the street, green infrastructure, improved freight loading zones, and consolidated space for a shared-use path.

Pearson's team met with residents and business owners — including during a walking and biking tour of the roadway — to explain the project's importance. "We started to be able to talk through shared visions using the context of this street, which I think is really critical," Pearson says.

A question of 'What do you value?'

While conducting the study, Hamidi and the team of investigators spoke with several departments of transportation and urban planners. A common theme was concern about liability risks if lanes widths were reduced and crashes went up.

"But they also mentioned that, in practice, justifying designing and implementing narrow travel lanes is challenging," Hamidi says. "Now, we have the data, and the data says narrower lanes are safer than wider lanes. So, how about we consider lane width reduction and make streets more livable, multidimensional, and safer for everyone?"

But that disconnect over safety is compounded by another challenge — getting decision makers to place that value above overall traffic efficiency.

Robert McHaney, AICP CTP, chair of the American Planning Association's Transportation Planning Division and chief of integrated planning at Texas-based The Goodman Corporation, says he advises his clients and policy makers — but does not prescribe answers.

"What do you value in your transportation infrastructure?" McHaney asks. "Is it moving from Point A to Point B really fast and efficiently? Do you value trying to develop a place where people want to walk, be, or hang out? The challenge is there is typically only enough right of way for achieving a few of the goals."

McHaney believes the study's recommendations are important for transportation planners to at least evaluate as the data may help get buy-in from stakeholders to make changes. He also thinks planners play a crucial role in that conversation.

"What does the community want to look like in the next 10 to 15 years?" he asks. "Because transportation can help drive that vision."

Jonathan DePaolis is APA's senior editor.