Technical Resources

Speed as a Safety Problem

Speed is defined as the rate at which someone or something is able to move or operate.

Speeding is traveling too fast for conditions or in excess of the posted speed limits.

Mobility is defined as the ability to move people and goods freely and easily in a transportation system.

Speed and Vision Zero

The goal of Vision Zero is to eliminate traffic related fatalities and serious injury by integrating safety into all work of transportation professionals. A core element of Vision Zero is creating safe speeds for a specific road context. The ITE Vision Zero initiative and Vision Zero Network What is Vision Zero both provide additional information, the mission of Vision Zero, and the impact speed has on creating safer roads.

To achieve the goal of Vision Zero, transportation professionals must design roads to provide safety for all functions and users, set safe speed limits for the context, and work collectively on proper enforcement and data collection measures to ensure desired speeds are achieved once a road is constructed and a speed limit is set. For vehicle to vehicle crashes, the likelihood of fatality increases as speeds increase. For crashes involving vulnerable road users, such as pedestrians and bicyclists, severe injury or fatality can still occur when a crash occurs at a low speed.

Speeds Relation to Safety

The speed at which a motorist travels on a given section of road is based on many factors. The motorist must take into consideration vehicle capability, roadway features, environmental conditions, surrounding context, presence of other road users, and most importantly, the speed limit. Speed becomes unsafe when a motorist travels too fast for conditions, but those conditions can vary based on road type, users and context. A transportation professional must design roads and set appropriate speed limits in order to create a safe environment for all users.

­­­­­Vehicle Speed comparison to chance of Pedestrian Injury and Fatality
Data source: US Department of Transportation, Literature Reviewed on Vehicle Travel Speeds and Pedestrian Injuries. March 2000.
Image credit: San Francisco MTA Vision Zero Action Plan, February 2015: https://view.joomag.com/vision-zero-san-francisco/0685197001423594455?short 

 

Speed is used as a measure for two key performance characteristics of roads—mobility and safety. Mobility is a factor of travel time, which may be minimized with higher speeds. Safety is associated with the reduction, if not complete absence, of crashes and fatalities. Safe speeds prioritize safety while balancing and taking into consideration the mobility needs of all road users, not just the movement of vehicles. For freeways and highways, desirable design features, access control, roadway alignment, roadside safety devices, etc., allow for higher speeds at an acceptable level of risk.  For urban and retrofitted streets, design complexities, such as right-of-way limits, land use context, presence of other road users, intersections, sight distances, etc., typically requires lower speeds for the safety of all road users. Transportation professionals should aim to implement an appropriate road design and speed limit for a given road context that creates safety without impacting mobility.

Field of vision based on speed of motorist
Original Source: Walkable City Rules, https://islandpress.org/book/
walkable-city-rules

 

Creating a safe road system is complex due to many possible crash causations, from driver, vehicle, roadway, and environmental factors. However, one causation is clear—crash severity increases as vehicle speed increases, especially for vulnerable non-motorized users. The AAA Foundation for Traffic Safety Impact Speed and a Pedestrian’s Risk of Severe Injury or Death study, published in 2011, developed risk estimates of severe injury or death for pedestrians struck by vehicles in the United States. This study has been recognized in various Vision Zero plans, such as San Francisco, CA, USA, to assist in determining the safest speeds for motorist vs. pedestrian scenarios.

Speeding and Road Users

Most studies have identified speeding as a contributing factor in the majority of crashes. The 2009  U.S. Department of Transportation, National Highway Traffic Safety Administration (NHTSA) Analysis of Speeding-Related Crashes: Definitions and the Effects of Road Environments publication outlines that “a crash is speeding-related if any driver involved in the crash is charged with a speeding-related offense or if a police officer indicates that racing, driving too fast for conditions, or exceeding the posted speed limit was a contributing factor in the crash.”

For a national perspective, NHTSA’s Fatality Analysis Reporting System (FARS) contains the most comprehensive data on speeding-related crashes and is therefore used as a baseline for speed management and crash related data. The NHTSA FARS data for 2016, showed that 27 percent of transportation fatalities is the U.S. were speed-related. The NHTSA FARS Speed Data Visualization Prototype is helpful to transportation professionals looking for data on speed related crashes by month, occupant, state, and other criteria.

Another important speed related criteria collected in FARS is by roadway types in the U.S. The percentage of speed-related fatalities is highest on local road types compared to all other road types. In general, local roads account for a majority of public land in our cities and have lower speed limits, which makes the high percentage of speeding-related fatalities as an important road type to consider in a speed management plan.

Number and Percentage of Speeding-Related Fatalities By Road Type

Road Type Number of Speeding-related Fatalities* Percentage of Speeding-Related Fatalities to Total Fatalities (%)
Interstate, Rural 544 24
Interstate, Urban 789 29
Freeway/Expressway 500 26
Other Principal Arterial 2513 23
Minor Arterial 1830 26
Collector 1948 29
Local 1552 37

Source: NHTSA FARS, 2016.

According to NHTSA FARS from 2010 to 2012, speeding-related crashes involving a pedestrian or bicycle occurred most often on local roads (23 percent), on roads with speeds between 30 to 35 mph (31 percent), and on non-intersection points on the road (54 percent).

The FHWA Integrating Speed Management within Roadway Departure, Intersections, and Pedestrian and Bicyclist Safety Focus Areas and Speed Concepts: Informational Guide, the NTSB Reducing Speeding-Related Crashes Involving Passenger Vehicles, and the GHSA Speeding Away from Zero: Rethinking a Forgotten Traffic Safety Challenge reports provide additional information, guidance, and critical data on designing safe roads and setting safe speed limits to reduce speeding related fatalities and serious injuries.