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How to influence road user behaviour by police enforcement?. A contribution to an answer from research. Fred Wegman SWOV Institute for Road Safety Research The Netherlands. “Car drivers defeated by speed cameras”. About SWOV Institute for Road Safety Research .

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how to influence road user behaviour by police enforcement

How to influence road user behaviour by police enforcement?

A contribution to an answer from research

Fred Wegman

SWOV Institute for Road Safety Research

The Netherlands


“Car drivers defeated

by speed cameras”

about swov institute for road safety research
About SWOV Institute for Road SafetyResearch
  • Independent, non-profit institute, founded in 1962
    • Aims to improve road safety by ‘evidence based’ knowledge
    • By carrying out research, and by
    • Knowledge dissemination to road safety professionals
  • Subsidy from Dutch Ministry of Transport and national and international contracts
  • Current staff: 60 ftu

structure of this presentation
Structure of this presentation
  • Why do we have road crashes, how to prevent them (best)?
  • How does police enforcement fit in a road safety strategy?
  • Why police enforcement should work to reduce the number of casualties?
  • How to set priorities for police enforcement?
  • Conclusions and recommendations


A. Why do we have road crashes

and how to prevent them (best)?

just one accident
…just one accident
  • 18 years old
  • Just passed his/her driving test
  • Saturday night
  • Drives his/her friends home
  • …from a party
  • Windy dyke
  • It’s raining
  • Misjudges a bend
  • Drives too fast
  • Trees alongside the road

  • A young, inexperienced driver
  • Distracted by the passengers
  • Driving at night
  • In the rain
  • With inappropriate speed
  • An unexpected sharp bend
  • Bald tyres
  • Trees in a bend
  • Don’t look for one single cause or one culprit!

traditional approach
Traditional approach
  • Define high risk groups/conditions and take cost-effective countermeasures, e.g.
    • Young novice drivers
    • Black spots
    • Vehicle inspection
    • Violators, recidivism
  • Certainly progress can be made in many countries, but we reach a stage that this will be less effective/efficient
  • We need a paradigm shift!
    • examples from three countries

reasons for fatal injuries sweden
Reasons for (fatal) injuries: Sweden
  • Excessive force: road user makes an error
  • Excessive risk: injured due to insufficient personal protection
  • Beyond system restrictions: violating regulationsSweden: two-thirds due to excessive forceSRA (

discussion on a paradigm shift in the uk
Discussion on a paradigm shift in the UK

Relative contribution

Driver failures:


Rod Kimber TRL (2003)

new road safety paradigm in nl
New road safety paradigm in NL
  • Road system is inherently unsafe
  • Human errors are and will always be made
  • Making human errors less likely by:
    • Eliciting the desired/safe behaviour
    • Making undesired error less likely
  • Forgiving human error by:
    • Providing an opportunity to correct an error
  • Reducing the consequences, once a crash has become inevitable, by system design based on ‘human tolerance’

sustainable safety

English version published in November 2006

  • Copies free downloadable from
Sustainable Safety
  • Philosophy developed in early nineties by SWOV
  • Basis of the Dutch road safety policy
  • Implementation since mid nineties
  • Update in 2006


B. How does police enforcement fit in

a road safety strategy?


Speed management

Current speeding


Speed indicators


Dynamic relations

Vehicles: top speed/

impact speed

Speed and

Road crashes

Attitudes to speeding

Speed and road design


Vehicle measures

Speed limits



enforcement in a road safety strategy
Enforcement in a road safety strategy
  • Using long term vision, system-wide strategies and targets + targeted programmes
  • Implement measures that are known to be correct and effective (and efficient)
  • Use effective delivery mechanisms (regionally and locally); look for partnerships
  • Compliance with road safety rules is a key safety strategy: enforcement!

evaluation of traffic law enforcement
Evaluation of Traffic Law Enforcement
  • Traffic Law Enforcement and risk of death(Redelmeier, Tibshirani and Evans, The Lancet, 2003)
  • Method: 8975 fatal crashes in Ontario, Canada (11 years), 21,500 driving convictions, case-crossover study
  • Findings: risk of a fatal crash in the month after a conviction was about 35% lower than in a comparable month with no conviction. No significant benefits after 3-4 months.
  • So: enforcement helps!

relationship between violations and risks
Relationship between violationsand risks?
  • The correlation between violation and crash records is positive, but small (r=.2)
  • The stability of individual records from one year to another is small
  • Violators will experience more crashes, but for individuals the prediction is useless
  • Previous violations better predict future violations and previous crashes predict better future crashes
  • A small group commit many violations, but account for a small fraction of recorded offences and crashes

violations and risks
Violations and risks
  • Many well known main risk factors (related to violations)
  • Speed, alcohol (+drugs), seat belts, child restraints, crash helmets, red light running, technical defects, commercial traffic, fatigue, aggressive driving/road rage, short headways, mobile phones, etc.

which violations to select to be targeted by enforcement

How to set priorities for police enforcement?

Which violations to select to betargeted by enforcement?
  • How do violations contribute to risk increase?
  • Which violations are enforceable?
  • Which effects of enforcement on behaviour?
  • Which effects of changed behaviour on crash reduction?
  • How to incorporate cost-benefit considerations in priority setting?

violations and risks24
Violations and risks
  • Most important violations according to their contribution to road accident fatalities in the Netherlands:
    • Speeding: nobody higher than existing speed limits: 20-25% less fatalities
    • Drinking and driving: nobody above legal limit: 20-25% less fatalities
    • And all the other violations?
    • Total: 50%?
  • Which priorities?

use evidence based knowledge
Use evidence based knowledge

speed management by enforcement
Speed management by enforcement
  • Speed cameras are more effective than physical policing methods
  • Speed cameras are more effective in reducing crashes inside urban areas than on rural roads
  • Fixed speed cameras are more effective in reducing speeds and crashes than mobile speed cameras
  • Speed cameras appear to have a distance halo effect
  • Physical policing in combination with randomization of police checks over road sections and times have a larger halo effects (minimally five times larger than that of speed cameras).

TRL review (Elliott and Broughton, 2005)

three questions of priority setting
Three questions of priority setting
  • Which violations to select to be targeted by enforcement?
  • Which effects are expected of the deployment of enforcement manpower + technology and which methods of enforcement to select?
  • What to learn form evaluation studies on enforcement?

distinguish three levels professionality
Distinguish three levels professionality

Police enforcement on three levels of professionalism

  • Ad hoc enforcement:
    • Short term operations (few weeks), few, specific locations and strong emphasis on catching offenders
  • Project bound enforcement:
    • Longer term planning (several months), often covers a certain route or number of locations and explicitly described aims, guided by evaluation efforts
  • Planned enforcement:
    • Annual plans, larger areas (regions, provinces), permanent operation, based on area-analysis, and evaluation results

general conclusions on priority setting
General conclusions on priority setting
  • A lot of knowledge is available
  • Available knowledge is not always used in enforcement practices
  • Still major questions on distance- and time halo effects
  • More knowledge needed when scaling-up results of experiments

deployment of enforcement manpower i
Deployment of enforcement manpower (I)
  • Nowadays based on: high number of crashes, high risks or high number of violations and judgement based on common sense about enforceability
  • Three approaches:
    • How to prevent per ‘unit of enforcement’ most fatalities/injuries (highest b-c ratio)
    • How to organise general deterrent best?
    • How to deal with hard-core behaviour and serious recidivism?

Not very well researched, space for improvement

deployment of enforcement manpower ii
Deployment of enforcement manpower (II)
  • My personal opinion
    • Rely on random selection of persons, locations, and times of deployment, thus avoiding a (predictable) pattern (general deterrence)
    • Combined with efforts to detect main/heavy/frequent violators (specific deterrence)
    • Balance?
    • Methods?

impact of enforcement on crashes i
Impact of enforcement on crashes (I)
  • Theoretical estimates based on enforcement inducing full compliance: 40-50%
  • Estimates based on empirical studies: about 10%
  • Estimates of some well implemented and sustained enforcement efforts: 20-25%

impact of enforcement on crashes ii
Impact of enforcement on crashes (II)
  • What to do to bring the 10% (observed effects) to 20-25% (potential effects)?
    • Better performance by the police: better use of knowledge, more funds, better organisation and management, better education, better motivation?
  • What to do to bring the 20-25% (potential effects) to 40-50% (full compliance)?
    • Better implementation of a road safety strategy: which interventions are effective, cost-effective, reduced return, acceptance by public and politicians?

conclusions and recommendations i
Conclusions and recommendations (I)
  • Non-compliance with traffic laws is massive in many countries, if not all, and is a major road safety problem
  • Non-compliance leads to additional deaths and injuries
  • Enforcement has a positive impact on compliance and is an important road safety intervention
  • Our theoretical basis of traffic enforcement is strong enough (through general deterrence and using specific deterrence, creating intrinsic motivation)

conclusions and recommendations ii
Conclusions and recommendations (II)
  • Enforcement should focus on those violations which can be prevented best by enforcement; best practices and knowledge transfer are needed
  • Enforcement should play a more integral role in road safety strategies instead of a stand alone role (not all violations should be prevented by enforcement only)
  • Scientific research can be instrumental to support enforcement authorities (by knowledge transfer and evaluation research)
  • A planned multi-year approach to enforcement should be set up, incorporating credibility, the whole enforcement chain and monitoring of results

conclusions and recommendations iii
Conclusions and recommendations (III)
  • More emphasis should be placed on the dose-response relationship of enforcement, also in research
  • Large and permanent increase in policing resources have a measurable positive effect on road safety
  • Investments in the quality of enforcement is needed to make it more effective, efficient, and credible;


  • Consider earmarking income from traffic tickets to enforcement activity (self-funding)


2020 Mortality rate

3 per 100,000?

Further progress, e.g. the Netherlands