Does 20mph make roads safer?

Eric BridgstockIndependent Road Safety Researcher

St Albans

National Road Safety Conference13 November 2013

Slide 4

What is safe/safer?

Safe = free from harm or exposed to an acceptable level of risk

Safer = less risk

Success measured by reduced casualtiesCasualties result from accidents

So what causes “accidents”?

Slide 5

The Recipe for an Accident

A collision/accident needs

hazard or hazardous condition AND

triggering event

This combination initiates an accident sequence, which will become a collision unless something can prevent it …

Slide 6

Hazards and Events

Hazards - driver or vehicle Other hazards - weather conditions, road layout,

other road users

Triggering events - misjudgement, poor observation, lapse of concentration, unsignalled manoeuvre, act of aggressive driving, vehicle failure, falling asleep

A collision can be prevented by – removing or mitigating the hazard–preventing the triggering event–mitigating the consequences (accident sequence)

Slide 7

Accident Sequence

Hazard

TriggeringEvent

Slide 8

Could a 20mph limit prevent a collision?

Speed limits encourage lawful drivers to drive at, or below, the prevailing speed limit

Is a lawful driver exceeding a speed limit a hazard? If they were, then lowering a speed limit would immediately make vehicles “hazardous” that were previously not hazardous – that is obviously illogical, since the number on a sign has negligible bearing on an accident scenario

Any moving vehicle is hazardous to some degree, and that hazard may vary with speed but there is no speed threshold that would be recognised as intolerable

20mph does not remove hazards (or have any measurable positive effect on them)

Slide 9

Could a 20mph limit remove triggering events?

No!But 20mph limit create hazards and triggering events: Pedestrians encouraged to feel safer, so take less

care when walking into the road Slow speeds can also reduce driver concentration

levels

20mph does not make roads saferit actually makes them LESS SAFE

20mph cannot mitigate or terminate an accident sequence - that involves driver skill, crash barriers, seatbelts, air-bags, crumple zones, ABS, etc. and even luck!