The language of road safety, explained
Road safety has its own vocabulary — drawn from engineering, public health, law and vehicle technology. This glossary defines the key terms used across Road Safety News and throughout the field. It is a living resource and grows over time. Suggest a term: Contact
A
ADAS (Advanced Driver Assistance Systems) Electronic systems that support the driver, such as autonomous emergency braking, lane-keeping assistance and adaptive cruise control. ADAS assists but does not replace the driver (see SAE Levels).
AEB (Autonomous Emergency Braking) A system that detects an impending collision and applies the brakes automatically if the driver does not react. One of the most effective crash-avoidance technologies; increasingly mandatory in new vehicles in many markets.
Alcohol interlock (ignition interlock) A breathalyser device wired to a vehicle’s ignition. The engine will not start unless the driver provides a breath sample below a set alcohol limit. Used in rehabilitation programmes for drink-driving offenders and in some commercial fleets.
At-grade crossing A point where a road, path or railway crosses another at the same level, rather than by bridge or underpass. Level crossings and pedestrian crossings are at-grade crossings.
B
BAC (Blood Alcohol Concentration) The amount of alcohol in a person’s blood, usually expressed in grams per decilitre (g/dL) or per mille (‰). Legal limits for drivers vary by country, commonly between 0.00 and 0.08 g/dL, often with lower or zero limits for novice and professional drivers.
Black spot (accident black spot / high-risk site) A location with an unusually high concentration of crashes, identified through crash data analysis. Black spot programmes target these sites with engineering treatments.
Blind spot An area around a vehicle that the driver cannot see directly or in mirrors. A major factor in crashes between trucks and pedestrians or cyclists, addressed through additional mirrors, cameras, sensors and direct-vision cab design.
C
Casualty A person killed or injured in a crash. Casualty statistics typically distinguish fatalities, serious injuries and slight injuries.
Child restraint system (CRS) Seats and boosters designed to protect children in vehicles, matched to a child’s size and weight. Correctly used child restraints dramatically reduce the risk of death and serious injury for young passengers.
Collision The preferred term (with crash) for a road traffic event involving impact. See the Road Safety News Style Guide for why we avoid “accident”.
Congestion charging Fees for driving in a defined urban zone, designed to reduce traffic volumes. Reduced motor traffic typically also reduces crash exposure.
Crash An event in which a vehicle collides with another vehicle, person, animal or object, or leaves the roadway. Used in preference to “accident” because crashes are predictable and preventable.
Crashworthiness A vehicle’s ability to protect its occupants during a collision, through structural design, crumple zones, restraints and airbags. Assessed by NCAP programmes.
D
Direct vision standard A requirement that drivers — particularly of trucks — can see vulnerable road users directly through windows rather than only via mirrors or cameras. Pioneered as a licensing standard for lorries in London.
Distracted driving Driving while attention is diverted from the road, most prominently by mobile phone use. Includes visual, manual and cognitive distraction; hands-free phone use still carries significant cognitive distraction.
DUI / DWI (Driving Under the Influence / Driving While Intoxicated) Legal terms, mainly used in North America, for driving while impaired by alcohol or drugs. Definitions and thresholds vary by jurisdiction.
E
E-scooter (electric scooter) A stand-up powered scooter, typically speed-limited, used privately or through shared rental schemes. A fast-growing category with evolving regulation on speed limits, helmet requirements, minimum age and where they may be ridden.
Exposure The amount of activity in which crashes can occur — for example, kilometres travelled or time spent in traffic. Risk comparisons are most meaningful when adjusted for exposure (see Rate).
F
Fatality (road death) A death resulting from a road traffic crash. The international standard definition counts deaths occurring within 30 days of the crash; some countries use different windows, which matters when comparing national statistics.
Fatigue (driver fatigue) Impairment caused by tiredness or sleep deprivation. Fatigue slows reaction times and can cause microsleeps; its effects on driving performance are comparable to alcohol impairment at moderate levels.
Forgiving road / forgiving roadside Road design that reduces the consequences of driver error — for example clear zones, safety barriers, and breakaway poles — so that a mistake does not become a fatal crash. A core concept of the Safe System approach.
G
Graduated driver licensing (GDL) A phased licensing system for new drivers, typically moving from supervised learning to restricted solo driving (limits on night driving, passengers or alcohol) to a full licence. Strong evidence shows GDL reduces young driver crashes.
Grade separation Separating traffic flows vertically — with bridges, flyovers or underpasses — so they do not cross at the same level. Eliminates conflict points, particularly for pedestrians crossing major roads.
H
Haddon Matrix A framework created by physician William Haddon Jr. that analyses crashes across three phases (pre-crash, crash, post-crash) and three factors (human, vehicle, environment). Foundational to modern injury prevention.
Helmet law Legislation requiring helmet use, most commonly for motorcyclists and moped riders, in some jurisdictions also for cyclists. Quality-standard motorcycle helmets substantially reduce the risk of death and head injury.
High-visibility clothing (hi-vis) Fluorescent and retroreflective clothing intended to make road users more conspicuous. A supplementary measure — not a substitute for safe infrastructure and vehicle speeds.
I
ISA (Intelligent Speed Assistance) Technology that uses GPS mapping and/or camera sign recognition to inform, warn or actively discourage the driver from exceeding the speed limit. Required in new vehicle types in the European Union.
iRAP (International Road Assessment Programme) A global charity that assesses and star-rates the safety of road infrastructure (1 to 5 stars) for vehicle occupants, motorcyclists, pedestrians and cyclists, and models the benefits of upgrades.
K
KSI (Killed or Seriously Injured) A combined casualty measure used widely in reporting and target-setting, capturing the most severe outcomes of road crashes.
L
Level crossing A point where a road crosses a railway at the same level. A high-severity crash location addressed through barriers, warning systems or grade separation.
M
Median (central reservation) The area separating opposing traffic flows on a divided road. Median barriers prevent head-on crashes, one of the deadliest crash types.
Micromobility Small, light, low-speed vehicles — e-scooters, e-bikes, shared bicycles — increasingly common in cities and increasingly prominent in casualty statistics and regulation.
Modal shift A change in the transport modes people use — for example from car to public transport, walking or cycling. Affects both individual risk and total crash exposure across a network.
N
NCAP (New Car Assessment Programme) Independent programmes that crash-test new vehicles and publish safety ratings, typically as stars. Regional programmes include Euro NCAP, Global NCAP, ASEAN NCAP, Latin NCAP and others. NCAP ratings have driven major improvements in vehicle safety beyond legal minimums.
P
Pedestrian crossing (crosswalk) A designated place for pedestrians to cross the road, which may be uncontrolled (marked only) or controlled (signals). Design quality — visibility, lighting, refuge islands, approach speeds — determines how much protection a crossing actually provides.
Post-crash care The emergency response, medical treatment and rehabilitation that follow a crash. One of the five pillars of global road safety action; faster, better trauma care saves lives after crashes occur.
R
Rate (fatality rate) Deaths expressed relative to a measure of population or exposure — most commonly deaths per 100,000 population, or deaths per billion vehicle-kilometres travelled. Rates, not raw counts, are the correct basis for comparing countries or time periods.
Roundabout A circular intersection where traffic flows around a central island. Well-designed roundabouts reduce fatal and serious injury crashes at junctions by lowering speeds and eliminating right-angle and head-on conflicts.
Run-off-road crash A crash in which a vehicle leaves the carriageway, often striking a tree, pole or barrier, or overturning. A leading crash type on rural roads, addressed by clear zones, barriers and rumble strips.
S
SAE Levels (of driving automation) The standard scale from Level 0 (no automation) to Level 5 (full automation) defined by SAE International. Levels 1–2 are driver-assistance systems requiring constant supervision; Level 3 and above are automated driving systems under defined conditions.
Safe System approach The international consensus framework for road safety. Its core principles: humans make mistakes; the human body has limited crash tolerance; responsibility is shared between road users and system designers; and roads, vehicles and speeds should be designed so mistakes do not cost lives. Vision Zero policies are built on Safe System principles.
School zone A designated area around a school with lowered speed limits and often additional engineering and enforcement, protecting children on their journey to and from school.
Serious injury (MAIS3+) Injury severity is increasingly defined using the Maximum Abbreviated Injury Scale; MAIS3+ denotes serious injury by medical criteria, allowing more consistent comparison than police-reported severity.
Speed camera (safety camera) Automated enforcement equipment that detects speeding vehicles. Includes fixed, mobile and average-speed (point-to-point) cameras. Robust evidence shows cameras reduce speeds, crashes and casualties at treated sites.
Speed limit The maximum legal speed on a road. Under Safe System principles, limits should reflect the crash forces the human body can survive: commonly cited survivable-speed benchmarks are 30 km/h where cars mix with pedestrians and cyclists, 50 km/h at urban intersections with side-impact risk, and 70 km/h where head-on collisions are possible on undivided roads.
Speed management The combination of limits, engineering, enforcement and vehicle technology used to keep traffic speeds safe. Small reductions in average speed produce disproportionately large reductions in deaths.
T
Traffic calming Physical design measures — humps, chicanes, raised crossings, narrowed lanes, modal filters — that reduce vehicle speeds and volumes, typically on residential and urban streets.
30 km/h zone (20 mph zone) An area-wide low speed limit for streets where motor vehicles mix with people walking and cycling, reflecting the survivable-impact threshold for pedestrians. A flagship measure of modern urban road safety policy.
V
Vision Zero A policy commitment, first adopted by Sweden in 1997, that no death or serious injury on the roads is acceptable, and that the transport system must be designed accordingly. Now adopted, in various forms, by countries and cities worldwide.
VRU (Vulnerable Road User) Road users without a protective vehicle shell — pedestrians, cyclists, motorcyclists and moped riders, along with users of wheelchairs and personal mobility devices. VRUs account for around half of all road deaths globally.
W
Whiplash A neck injury caused by rapid head movement, typically in rear-end collisions. Common, costly, and mitigated by well-designed head restraints and seat structures.
This glossary is maintained by the Road Safety News editorial team and updated as terminology evolves. Definitions draw on international usage by road safety authorities, research bodies and standards organisations. You are welcome to link to this page; to republish definitions, please contact us.
