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