California Safe Speeds Toolkit: Glossary


85th percentile speed: “the speed at or below which 85 percent of the drivers travel on a road segment.” (FHWA n.d.(link is external))

Active Transportation: “human-powered mobility, such as biking, walking, or rolling” (USDOE n.d.(link is external))

Complete Streets: “streets designed and operated to enable safe use and support mobility for all users. Those include people of all ages and abilities, regardless of whether they are travelling as drivers, pedestrians, bicyclists, or public transportation riders” (USDOT 2021(link is external))

High injury network: a collection of streets designated by a municipality as having relatively high incidence of transportation-related crashes resulting in serious injury or fatality.

Prima facie speed limits: a default speed limit that applies under conditions when a posted speed limit is not determined based on an engineering and traffic survey

Safe speeds: "A safe speed is a vehicle speed that reduces system kinetic energy and accommodates human injury tolerance." (FHWA 2022(link is external))

Safe System: “approach to roadway safety envisions eliminating fatal and serious injuries for all road users by creating a transportation system that accommodates human mistakes and keeps impacts on the human body at tolerable levels” (GHSA n.d.(link is external))

Speed creep:  a phenomenon of ever-increasing speed limits because faster measured driver speeds require increases to speed limits and then higher speed limits prompt motorists to drive faster, again prompting a further increase to speed limits

Vision Zero: “a strategy to eliminate all traffic fatalities and severe injuries, while increasing safe, healthy, equitable mobility for all.” (Vision Zero Network n.d.(link is external))

Vulnerable road users: "non-motorised road users, such as pedestrians and cyclists as well as motor-cyclists and persons with disabilities or reduced mobility and orientation". (European Commission: Mobility and Transport n.d.(link is external))


This Toolkit does not constitute a standard, specification, or regulation. This Toolkit is not intended to replace the existing Caltrans mandatory or advisory standards and is not intended to be a substitute for engineering knowledge, experience or judgment. This Toolkit promotes better access to helpful information and concepts from various agencies and organizations. UC Berkeley and Caltrans acknowledge the existence of other practices and provide this Toolkit for reference and direction for those responsible for making professional engineering or other design decisions. 

Please be aware that neither UC Berkeley nor Caltrans is the author of the external sources contained in this Toolkit and are therefore not responsible for errors or omissions that may be contained within those external sources. The references contained in this Toolkit are provided for the user’s convenience. The content or information in such references is the sole responsibility of their respective authors and/or owners.