Can slimmer roads slim traffic too?

    A typical road diet might narrow a four-lane parkway to three lanes, with one lane for each travel direction, a two-way left turn lane in the center, and bike lanes.

    Can narrowing a road truly improve the experience for all users? From a safety perspective, the answer is fairly easy to intuit. Narrower roads necessitate lower vehicle speeds and proportionately lower the risk and severity of accidents. A 2010 Federal Highway Administration (FHWA) study, "Evaluation of Lane Reduction ‘Road Diet’ Measures and Their Effects on Crashes and Injuries" (, found that road diets, which narrow roads by reducing the number of traffic lanes, can decrease crash frequencies by an average of 29 percent, an improvement that can be attributed to dedicated turn lanes and reduced overall travel speeds. Vehicle users tend to cry foul at this point, objecting that road diets add yet another obstacle to their commute. Counterintuitive as it may seem, however, narrower roads and slower speeds don’t always mean a slower commute.

    As an example, consider a typical four-lane roadway. Four-lane roads have little room to accommodate bicycle users and allow for higher speeds, which can be unsafe for all user groups. A typical road diet might narrow a four-lane parkway to three lanes, with one lane for each travel direction and a two-way left turn lane (TWLTL) in the center. For non-vehicle users, the benefits of this slim-down are fairly obvious. The extra space gained by removing one lane can be used for bike lanes and sidewalks, pedestrians can enjoy narrower crossing distances, and all user groups can feel safer as vehicle speeds slow. Additionally, the road diet can free up space for amenities such as bus stops or on-street parking. All of this can be added without infringing on adjacent property, which can get very complicated very quickly.

    For vehicle users, the safety benefits of a road diet are also fairly obvious – slower speeds tend to decrease the frequency and severity of accidents, as does the TWLTL, which helps to minimize turning conflicts. What may be less intuitive is that narrower roads can improve traffic flow. On a four-lane road with no central turn lane, vehicles can travel at higher speeds but also have to stop more often to avoid turning cars. After a road diet, with three lanes including a shared turn lane, vehicles will travel more slowly but at more consistent speeds with less frequent stops for turning vehicles. Because there is less variation in speed, traffic volume can move through the area more quickly and overall congestion is minimized. So, the initial objections of vehicle users might not be valid – in many cases, road diets can actually improve the speed of their commute.

    However, road diets are not a good solution for every corridor; there are certainly some roads where a road diet would do little more than create a major traffic headache. Planners must consider the average daily traffic that a corridor experiences. If a roadway sees 18,000 to 20,000 vehicles per day or more, narrowing the road may do more harm than good. Below that range, however, a roadway might be a good candidate for a road diet. Public transportation routes also should be taken into account, especially on roadways with heavy bus traffic. Too frequent bus stops can cause more congestion on narrower roads and can negate the positive effects of a road diet. To avoid this, planners should analyze traffic pace, space transit stops appropriately to decrease congestion, and, if necessary, install designated bus pull-offs to remove stopping vehicles from traffic flow.

    With such planning, road diets can be a valuable tool for creating more complete streets with benefits ranging from increased safety and usability to improved community health, as more users are able to be more active on city corridors. The saying "less is more" might be a tad overused, but with road diets, I truly believe that to be the case. Implemented appropriately, less road surface means more pedestrian and cyclist activity, more throughput at slower speeds, and safer travel for all user groups. That’s something worth dieting for.

    For additional information and resources, see the Federal Highway Administration’s "Proven Safety Countermeasures" website at

    Jonathan D. Henney, AICP, ASLA, is a principal with Gresham, Smith and Partners (GS&P) and manages the land planning function of the firm’s Louisville, Ky., office. He has more than 30 years of experience in community planning projects, corridor studies, multi-modal and site design for commercial, residential, office, and mixed-used developments.