Curb Extensions and Modifications

Purpose

Shorten crossing distances, reduce vehicle turning speeds, improve visibility and sight distance, and increase pedestrian comfort at intersections.

Description

A wide curb radius allows drivers to turn right at high speeds and increases risk of collisions with pedestrians. Reducing the corner radius encourages drivers to slow down to take a sharper turn. Reducing the corner radii also reduces crossing distance for pedestrians and provides flexibility for curb ramp placement. Corners can be modified by adding curb extensions or truck aprons.

• Curb extensions can be implemented either (1) with pavement markings or flex posts for a low cost, temporary solution, or (2) using concrete to permanently convert the space to sidewalk area.
• Curb modifications help to change the geometry of an intersection or roadway and may serve to remove slip lanes, which reduces pedestrian conflicts.
• Curb extensions that are installed using tactical materials may integrate asphalt art (Arlington Community Canvas), which adds a visual element to the curb modification and reenforces that the area is pedestrian space rather than part of vehicle travel lanes or parking/loading space.
•  Truck aprons are low-profile, mountable curb extensions. They can be used in areas where a tighter curb radius is desired to slow turning vehicles, but where turning movements for large vehicles such as trucks and buses need to be accommodated.

Safety Benefits

• Shorten crossing distance.
• Increase visibility between drivers and pedestrians.
• Narrow the roadway to reduce through vehicle speeds.
• Reduce vehicle turning speeds, reduce impact forces, and increase driver yielding to pedestrians. Prevent drivers from parking near or on the crosswalk.
• Expand waiting areas for pedestrians waiting to cross.
• Provide additional space to improve accessibility, convenience, and comfort of transit stops.
• Studies in Arlington showed that before installation of the curb modifications, 35% of drivers were turning at speeds less than 10 mph and this percentage increased to 49% after installation.

Street Types and Context

Applicable Street Types

All street types.

Other Location Guidance

Curb modifications can be made everywhere from a mid-block crosswalk to a large, signalized intersection. Curb extensions can be built in all-day parking lanes or wide shoulders.

Primary User Groups

Key Implementation Considerations

• Curb modifications should not extend into travel lanes or bicycle lanes.
• Where bicycle lanes are present, consider other treatments that allow for better separation of bicycles and vehicles, such as protected intersections or bus boarding islands.
• Travel lane or turn lane removal or narrowing must be analyzed prior to considering using this tool to accommodate a smaller curb radius. 
• Curb extensions can be used at intersections with low truck or bus turning volumes. The goal should be to make the intersection as compact as possible while still allowing larger vehicles to navigate the turns. Truck aprons can be used to facilitate turning movements for large vehicles.
• Corner radii that are too small may encourage vehicles to drive over the curb and onto sidewalks or raised bikeways.
• Curb extensions can require modifications to or relocation of drainage structures. 
• A longer timeframe may be needed if curb modifications are included in a capital or redevelopment project. 

Expected Crash Reduction

Nearly 50% based on engineering judgement. A CMF has not yet been determined; initial research indicates this treatment may be effective at increasing driver yielding and improving pedestrian safety (Johnson et al. 2005; Thomas et al., 2016).

Added asphalt art at crossings reduced the rate of crossings involving a crash potential conflict by 25%. Additionally, drivers yielding immediately to pedestrians was 27% more frequent after art was installed, and the frequency of no vehicles stopping for the pedestrian reduced by 27%. (Bloomberg, 2022)

Cost

Costs vary depending on design, site conditions, and materials. Designs with only markings and/or flexible delineator posts are lower-cost alternatives but require ongoing maintenance.

Timeline

Less than one to three or more years, depending on complexity.

References

• NACTO Urban Street Design Guide
• FHWA Guide for Improving Pedestrian Safety at Uncontrolled Crossing Locations
• PEDSAFE Pedestrian Safety and Countermeasure Selection System
• Public Right-of-Way Accessibility Guidelines