Researchers at the Johns Hopkins Bloomberg School of Public Health’s ‘Bloomberg American Health Initiative’ analysed lane widths in seven US cities – Dallas, New York City, Philadelphia, Salt Lake City, Miami, Denver, and Washington DC –and found that even slightly narrower lane widths are, in many cases, safer than wider ones.

Local and state departments of transportation have long favoured lane widths between 11 and 12 feet for city streets with the assumption that the extra space is safer and can accommodate pedestrians and cyclists. However, in an analysis of 1117 streets in the seven cities, the authors found that reducing city traffic lane width to 9 feet, especially in traffic lanes with speed limits up to 35 miles per hour, could help reduce traffic-related collisions. Reducing lane width would also create more space for other safety and liveability features, such as bicycle lanes and wider sidewalks (pavements).

The authors found that the number of vehicle crashes do not significantly change in streets with a lane width of 9 feet compared with streets with lane widths of 10 feet or 11 feet. There are significant increases in crashes – about 1.5 times higher – when the lane width expands from 9 feet to 12 feet.

The researchers also found no significant changes in car crashes with wider traffic lanes in a speed limit zone of 20–25 miles per hour. However, traffic lanes with 10-, 11-, and 12-foot lane widths have significantly higher crashes than lanes that are 9 feet wide in zones that are 30–35 miles per hour.

“Our study of city lane widths found that contrary to the current thinking, wider lanes in urban areas can lead to a higher number of crashes and ultimately fatalities,” said Shima Hamidi, PhD, Bloomberg assistant professor of American health, and director of the Center for Climate-Smart Transportation at the Bloomberg School. “What if we can narrow lanes without sacrificing safety, and how can we best use the additional space in the existing infrastructure? That’s what we want to know.”

Major cause of death

Road traffic collisions are a leading cause of death for people aged 1 to 54 in the US. Pedestrian fatalities, especially in urban areas, jumped by more than 40 per cent from 2010 to 2018, while 2020 was the deadliest year for pedestrians in 40 years. A lack of pedestrian and bicycle infrastructure, along with street design, specifically travel lane width, are factors that contribute to the high burden of fatalities.

For the study, the researchers identified 7670 street sections – excluding highways and interstates – in the seven US cities that had similar average daily traffic and more than 21 key roadway design characteristics, such as number of through lanes, presence/types of medians, and on-street parking. The researchers then randomly selected about 15 per cent – 1117 – of street sections for the analysis.

Using Google satellite imagery and information from city and state departments of transportation, the researchers analysed the relationship between lane width and the number of crashes that occurred in each road section from 2017 to 2019. They looked at lane width; number of lanes; median types; shoulder widths and types; presence of sidewalks; bus stops; on-street parking; turn lanes; number of intersections; speed limits; and roadside details, such as buildings and trees.

The report also sets out several policy recommendations for possible lane-width reduction projects with a focus on urban settings. The recommendations are based on the report’s seven-city analysis and interviews with officials in department of transportation offices in five states: Vermont, Florida, Colorado, Delaware, and Oregon.

Recommendations include:

• street sections with lane widths of 11, 12, or 13 feet in urban settings with a 20–25 or 30–35 miles per hour speed limit that do not serve as a transit or freight corridor should be considered first for lane-width reductions;
• decision-makers should set the standard lane width at 10 feet in low-speed urban settings and provide justification for wider lanes;
• city and state departments of transportation should establish a context-appropriate speed before determining lane width;
• engineers and city planners should prioritise inclusive street design rather than driving speed and functionality; and
• city and state departments of transportation could pair lane-width reduction projects with the launch of a lane-repurposing programme to add a bike lane or wider sidewalk.

“Lane-width reduction is the easiest and most cost-effective way to accommodate better sidewalk and bike lanes within the existing roadway infrastructure,” said Hamidi. “Narrower lanes ultimately minimise construction and road maintenance, and also reduce environmental impacts.”

The report, ‘A national investigation of the impacts of lane width on traffic safety’, is being released ahead of the sixth annual Bloomberg American Health Summit in Baltimore, Maryland.

The project was supported with funding from the Bloomberg American Health Initiative.