SWOV Monitoring the drink-driving trend - Tips and tricks on how to design a roadside survey
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SWOV
Monitoring the drink-driving trend
Tips and tricks on how to design a roadside survey
Dr. Sjoerd Houwing
SWOV Institute for Road Safety Research
Background
• Drink driving is an important factor in road
safety crashes
• 20%-28% of all road fatalities in Europe are
alcohol related
• Drink drivers use their seat belts less and are
more often speeding than sober drivers
IBSR, April 23rd 2015
Background
Combating drink driving is one of the pillars
of a succesful road safety policy!
IBSR, April 23rd 2015
Who is this drink driving offender?
No typical drink driver exists, but relatively often:
• Male (18-35),
• Unemployed or labour worker,
• Single or divorced/separated,
• Prior driving offences or other criminal records.
IBSR, April 23rd 2015
Why do we monitor drink-driving?
• To get information on the prevalence of drink driving
• To monitor the development of specific driver groups
• To evaluate the effectiveness of measures
– before vs after
– study group vs reference group
– study area vs reference area
IBSR, April 23rd 2015
Best indicators in theory
• Direct crash-based indicators
• Observed safety performance indicators
• Self reported safety performance indicators
• Indirect crash-based surrogate indicators
• Non-crash-based surrogate indicators
IBSR, April 23rd 2015
Best indicators in theory
• Direct crash-based indicators
• Observed safety performance indicators
• Self reported safety performance indicators
• Indirect crash-based surrogate indicators
• Non-crash-based surrogate indicators
IBSR, April 23rd 2015
Roadside surveys ‘gold standard’
• Test all active road users
• For all psychoactive substances
• During all times of the day and all days of the week
• Collect as much additional information as possible.
STOP, have you
been drinking?
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Roadside surveys in practice
IBSR, April 23rd 2015
Some examples of limitations
Practical limitations
• Restrictions in budget, time, and manpower
• Limited cooperation police
Legal limitations
• No random breath testing allowed
• No interference of standard police procedure
IBSR, April 23rd 2015Step 1: check legal issues and willingness
of the police to cooperate
• Study design should be according to the legislation
and ethical approved
• Participation police is very important
• Alternatively, breath testing on a voluntary base
(e.g. at parking lots and gasoline stations).
- Participation rates
- Safe working conditions
- Ethical issue: How to respond to drivers
with BACs over the legal limit?
IBSR, April 23rd 2015Step 2: Conduct a power study
Conduct a power study to determine how many
samples you will be needing to answer your research
questions.
You will probably need information from previous
studies or expert opinions to fill in the expected
values that are required for the power analyses.
IBSR, April 23rd 2015Step 3: Design your study in full
detail
Selection bias is most important issue in roadside
surveys on drink-driving
• Representative selection of research sites
• Random testing
• Minimize non-response, especially selective non-
response
• Be aware of possible confounding (stratified
sampling, adjustment in analysis)
IBSR, April 23rd 2015Taking a representative sample
Design a grid of locations that is representative for
traffic in these time periods (e.g. main roads in urban
and rural areas)
Locations should have save working conditions and
limited possibilities for drivers to avoid the
police
IBSR, April 23rd 2015Taking a representative sample
Avoid survey sessions at locations or days that may
not be representative for normal traffic conditions
during the study period (e.g. festival or holidays)
Avoid selective testing based on type of car or type of
driver (e.g. use random sampling techniques)
IBSR, April 23rd 2015Selection of additional information
• Gender and age
• Type of vehicle Either all drivers that are
• Passengers tested or those drivers
• Nationality who have used alcohol
• Novice driver
• Location of drinking
• Recidivism Drink driving offenders
IBSR, April 23rd 2015Step 4: Conduct a pilot study
Conduct a pilot study to check if all works out in
practice like you have planned.
IBSR, April 23rd 2015Step 5: Start your study
Adjust the study protocol if necessary or, if all went
well, start directly with the main study.
IBSR, April 23rd 2015Case studies: Canada
Roadside survey to measure effect of new measures implemented in
September 2010
Design: Before-after study, 21:00-03:00 in Wednesday through Saturday
nights
Target N= 2,500 per year, based on a power study (95% C.I. of ± 1.1%).
Police officer directs next driver to research location, but researcher tests
for psychoactive substances (alcohol and drugs).
Incentive for drug test: 10 dollar coupon for gasoline, taxi or designated
driver service was provided for drivers over the legal limit
Response rate 90% alcohol breath tests and 70% drug tests in oral fluid
IBSR, April 23rd 2015Case studies: Netherlands
national study
Roadside survey to measure the development of drink driving
and the effect of new measures (1970-present; annual or each
2 years)
Design: Friday and Saturday nights, 22:00-04:00.
Target N= 25,000 per year, 2 or 3 sessions per region
(sufficient number for each region, no power study).
Data drivers recorded by researchers for all drivers (age,
gender, years of possession driver license, location of drinking,
passengers).
IBSR, April 23rd 2015Case studies: Netherlands regional
study
Measure the development of drink driving on a regional level
(Province of Zeeland since 2003, other regions may follow)
Design: 21:00-04:00 in Saturday nights
Target N= 5,000 per year, to collect sufficient data (power study).
Survey sessions spread over year: One session per subregion (3)
in each quarter of the year.
Police officer collects additional information from offenders only
(age, gender, passengers, novice driver, location of drinking,
recidivism).
IBSR, April 23rd 2015Case studies: Belgian national
study
Roadside survey to measure the development of drink driving and
the effect of new measures (2003-present; each 2 or 3 years)
Design: 4 time periodes: week and weekend days 06.00-22.00,
week and weekend nights, 22.00-0200.
Target N= 10,000 -12,000 per year, 2 or 3 sessions per zone
(sufficient number for each time period, power study).
Logistic regression used to e.g. estimate influence of variables on
alcohol use. Traffic counts to determine share of drink driving.
Data car drivers recorded on (age, gender, driver license,
location of drinking, passengers (+age and gender).
IBSR, April 23rd 2015Case studies: DRUID-study
Roadside survey to measure the share of drink
driving per country
Design: 13 EU countries, 23 psychoactive substances, all days
of the week and times of the day
Target N= 1,000-5,000 per country, based on a power study, in
total 50,000 drivers.
Procedure of stopping and testing varied between countries
due to legal and medical ethical limitations
Incentives varied per country, Large variation in response rate,
some countries suspected of selective testing
IBSR, April 23rd 2015Conclusions (1)
• Participation of police and legal limitations
very important factors when designing your
study
• Find a balance between practical and
theoretical optimal design, use weight factors
and power studies for assistance
• Conduct a pilot study
IBSR, April 23rd 2015Conclusions (2)
• Don’t reinvent the wheel: use the experience from
previous studies (both national and international)
IBSR, April 23rd 2015Thank you for your attention!!
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