Design Speed

Design speed is the target speed at which drivers are intended to travel on a street, and not, as often misused, the maximum operating speed. Actively designing for target vehicle speeds is critical to safety. Changing a street’s design results in behavior changes. Practitioners must manage speeds by setting clear expectations for drivers. The level of walking, cycling, and activity, as well as the degree to which modes are mixed or separated, is the critical determinant of a safe vehicle speed. Reducing vehicle speeds opens up a range of design options that allow a street to function and feel like part of a city, rather than a highway. Designers must not use highwaybased design speed practices in urban areas. Instead, they must be proactive in limiting vehicle speeds, providing frequent pedestrian  crossings, limiting the number and width of lanes, using low speeds for turn radii, and introducing trees and furnishings.

Conventional practice designates a design speed higher than the posted speed limit to accommodate driver errors. But, in fact, this practice only encourages speeding and increases the likelihood of traffic crashes, fatalities, and injuries.

A proactive approach selects a target speed and uses design to achieve that speed, guiding driver behavior through physical and perceptual cues. These cues include narrower lane widths and tighter curb radii, signal progressions, and other speed management techniques. Using lower design speeds in street design reduces vehicle speed and speed variation, providing safer places to walk, cycle, drive, and park.

The design speed for urban areas should not exceed 40km/h, with exceptions for specific corridors. To determine appropriate design speeds other than 40km/h, consider the multiple safety, health, mobility, economic, and environmental goals.

Speed, Severity, and Frequency

The most effective way to reduce fatalities and severe injuries on streets is to reduce vehicle speeds.¹ The vast majority of people killed in traffic are struck on streets with high speeds, even though those streets represent only a small portion of a city’s total activity and movement.

Speed is the primary factor in crash severity and the likelihood of a crash occurring. Increased speeds result in longer reaction times, a narrower cone of vision,² and increased stopping distances while providing less time for others to react. An increase in average speed of 1 km/h results in a 3% higher risk of a crash and a 4–5% increase in fatalities.³

Speed differential is also a critical component of safety. People walking or cycling are placed at great risk when faced with motor vehicles turning across their paths, at a speed much faster than their own. Keeping design speeds low on streets where cycles, cars, trucks, or buses share a lane or street reduces crash risk, and the likelihood of serious injury or death. A design assumption that modes are separate can become dangerous when user expectations vary. The safest streets match the degree of mixing with an expectation of mixing, using design to communicate the specific conditions.

Target Speeds and Context

Critical Guidance

Do not design streets for speeds higher than the posted limit.

Establish the target speed based on all users and not just drivers. Develop a realistic assessment of the way the street is used, and account for the immediate context and municipal safety goals. Design streets to constructively guide driver behavior, discouraging speeds above the target speed and promoting the safe mixing of multiple modes.

Set speed limits at the target speed if possible. If statutory speed limits are  higher than safe urban speeds, set the design speed below the speed limit. Make speeding uncomfortable through design and operational techniques.

Target speeds must, in all circumstances and on all streets, allow people to walk along and to cross streets without substantial risk of being injured by vehicles. They must provide drivers with adequate time and distance to avoid striking pedestrians crossing the street.

Do not use target speeds of 60 km/h or higher. These speeds endanger safety on urban streets and are reserved for limited-access highways.

Recommended Guidance

Desired vehicle speed should be achieved by choosing street sections that encourage safe speeds. Keep the total number of through-traffic lanes to a minimum. Choose a small radius for turns, use signal timing to promote low speeds, and apply speed management techniques where the street section does not prove to be sufficient.

Where speeds greater than 40 km/h are allowed create a physical separation between vehicles and vulnerable users such as pedestrians and cyclists. Parked vehicles, built medians, buffers, or other vertical elements may be used as a barrier, though frequent opportunities for safe crossing must be provided. These should be ideally located at intervals of 80–100 m, and not more than 200 m. See: Speed Management.

Limit the speed differential within and between modes. If people walk in the same space as motorists, use a speed of 10–15 km/h. If pedestrians routinely cross the street mid-block, away from formal crossings, select a target speed of 20 km/h or less. If cyclists are mixed with motorists, but pedestrians do not share the same space, use a speed of 30 km/h or less, even at low traffic volumes. These speeds are also consistent with bus traffic.

Default design speeds based on street types or zones can be established as a starting point. Above 40 km/h, the use of specific engineering action, such as signalization and other conflict management techniques, is recommended to create basic conditions for safe use across all modes.

Urban streets can rarely accommodate pedestrians and cyclists safely when prevailing speeds are 60 km/h or higher. If speeds cannot be reduced, high-quality walking and cycle facilities should be provided at grade, with robust protection such as parallel parking, trees, and medians. Do not use techniques that discourage pedestrian activity and destroy or limit the street’s economic functions and social activity.

Foster a safety culture. Publicize and regulate the speed limit with signage, markings, and public information campaigns, and routinely enforce the speed limit. Full-time electronic enforcement by radar-enabled license plate readers or speed cameras and moderate fines are more effective and equitable than manual enforcement with high fines.