A road diet may take the form of a completely redesigned street with new infrastructure—like landscaped medians, expanded sidewalks, and converted lanes—or something as simple as restriping the road to add a bike lane or change the formation of parking. Below is a glossary describing some of the most common road diet strategies.
Bike lanes encourage biking, and can improve safety. Bike infrastructure ranges from lanes that are physically separated from motorized traffic by parking or other barriers to simple ‘sharrow’ markings painted on the street. The most common bike lanes on road diet projects are painted lanes on the edge of the road.
Many road diet projects take place on active bus routes. Especially where there is only one dedicated moving lane, it is important to create a space exclusively for buses at bus stops to avoid negatively impact other traffic when they stop to pick up passengers, and to ensure they are able to load and unload quickly. Where there are two moving lanes in a given direction, the lane closest to the curb near the bus stop can be made exclusive to buses as a turnout. A bus turnout can also take the place of parking. Alternately, a bus bulb can extend the sidewalk to the moving lane to speed boarding and minimize delays at bus stops.
In areas without driveways where lanes have been eliminated, it may be helpful to the adjacent businesses and residents to have designated drop-off curb space in the parking lane for some or all of the day to mitigate the potential disruption and ticketing of double-parked vehicles.
Landscaping, such as trees and other greenery, can make the street more appealing for all users, and can be used in tandem with other treatments to break up concrete's monotony, to attract pedestrians, and ease burdens on drainage systems by diverting storm water. Street trees in particular have a host of potential benefits, including traffic calming.
Lane directions can be reconfigured to support road diet goals, by changing a two-way street to a one-way street, or a one-way to a two-way street. Converting a two-way to a one-way street can free up space, either by eliminating a lane or simply because less street width is needed for two lanes going in the same direction. Conversely, converting from a one-way to a two-way street can be an effective traffic calming measure, slowing drivers down by reducing lane widths and demanding more cautious driving.
Many road diet projects narrow driving lanes. Research shows that narrower lanes reduce driving speeds, and the space recovered by narrowing lanes can be used for other purposes, such as additional sidewalk space, bike lanes, a plaza, or a median.
On the other hand, some projects reduce the number of driving lanes, but increase the width of the remaining lanes in order to create room for vehicles to pass in case of bus stops or double parking, or to make room for people bicycling.
On streets with relatively long distances between intersections, it is sometimes important to add a midblock pedestrian crossing to improve pedestrian safety. The crossing provides easy access between pedestrian attractions on either side of a street, like public spaces, retail, schools, or bus stops. Midblock crossings can be regulated with crosswalks, yield signs, stop signs, or traffic signals, depending on the amount of pedestrians and vehicles traffic. A midblock crossing is safer with a pedestrian refuge island (described below), particularly if the street has two-way traffic. Crosswalks can be made more visible and aesthetically appealing with high visibility paint and designs, or materials like bright stone work or asphalt printings.
Neckdowns (also known as curb extensions or bulbouts) are extensions of the curb that increase pedestrian space and shorten street crossings. They are commonly used for pedestrian crossings at intersections and midblock. Neckdowns protect pedestrians by slowing traffic, especially as drivers make turns, and by increasing pedestrians' visibility of drivers and reducing crossing distances. They make it especially easier and safer for slower pedestrians, like small children and the elderly, to cross the street.
As an integral part of the street, parking reconfiguration can play an important role in a road diet. In places with newly-freed up street space, parallel parking can be converted to diagonal parking, thus increasing the number of parking spots. These changes can encourage passing drivers to stop on main streets and improve safety for residents. Parking can also be used to protect bicycle lanes. In other projects, some parking may be removed to improve intersection safety or to create space for other improvements, such as converting a parking lane into a pedestrian walkway or bicycle lane.
Some streets have counterproductive or underutilized lanes for vehicular traffic, or have more parking than is necessary. There are often more beneficial uses for this public space. In many cities with large numbers of pedestrians and office workers, business improvement districts have worked with local government to convert this type of space into plazas or parklets. Times Square in New York City is one prominent example.
Pedestrian islands are places for pedestrians to stop while crossing the street. They are often located in the median of the street or sometimes between a bike lane and vehicular lane. They increase pedestrian safety and visibility much like a neckdown (described above), by reducing the distance that a pedestrian has to cross through moving traffic, and by allowing a slow-moving pedestrian to split a crossing into segments. They have been shown to reduce pedestrian injuries at intersections. Road diet projects may install new pedestrian islands or medians, or expand the width or length of existing ones.
A median is a space between opposing travel lanes in the middle of a road. It can be hard infrastructure raised from the street, sometimes landscaped, or it can simply be marked space in the road that separates traffic beyond a double yellow line. Sometimes a median goes from crosswalk to crosswalk, splitting the street the whole way. Other times, the median ends some distance from the intersection to give vehicles a place to wait to turn left without delaying the vehicles that are going straight through the intersection. Medians in the center of the block can eliminate midblock turns across traffic as drivers attempt to enter driveways on the other side of the road. Both pedestrian islands and medians benefit driver and pedestrian safety.
Roundabouts are circular intersections that are installed in place of stop signs or traffic signals. They allow traffic to safely maneuver through the intersection without coming to a complete stop or waiting for a red light. Research has shown that roundabouts can improve safety and mobility for all users, often at a lower cost than traffic signals. According to the Federal Highway Administration, roundabouts reduce crashes by 35%, injury crashes by 76%, and fatal crashes by 90-100%, compared to previous configurations. Roundabouts often reduce congestion during rush hours by allowing a continuous flow of traffic, and can also provide other benefits, such as an opportunity for landscaping. Roundabouts often cost about the same as traffic signals for installation, but have lower long-term maintenance costs. Each intersection should be separately examined, however, to ensure that the roundabout will function well.
A road diet may include adding sidewalks to streets that lack them or increasing sidewalk widths, and can accompany more general improvements to materials, landscaping, or lighting. Improving pedestrian infrastructure encourages walking, and can help generate room for other activities and infrastructure that encourage a sense of place.
A two-way or dual left turn lane gives drivers dedicated space to make left turns by creating a center lane exclusively for drivers with this intention. Two-way left turn lanes often replace what were two vehicle lanes (one in each direction) that could be used by both through traffic and left turning traffic. By separating left-turning vehicles from through traffic and making left turns more predictable, these lanes can decrease accidents while maintaining similar traffic volumes and point-to-point times.
Road diets sometimes convert lanes previously dedicated to vehicles to a new purpose like a bike lane, expanded sidewalk space, or a median to help make crossing the street safer and easier. Lane conversions also minimize lane changes and reduce speeds, making the street a safer place for everyone.