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Searching for causal effects of road traffic safety interventions: applications of the interrupted time series design
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Environmental and Life Sciences. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Centre for Public Safety.ORCID iD: 0000-0002-1189-9950
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Traffic-related injuries represent a global public health problem, and contribute largely to mortality and years lived with disability worldwide. Over the course of the last decades, improvements to road traffic safety and injury surveillance systems have resulted in a shift in focus from the prevention of motor vehicle accidents to the control of injury events involving vulnerable road users (VRUs), such as cyclists and moped riders. There have been calls for improvements to the evaluation of safety interventions due to methodological problems associated with the most commonly used study designs. The purpose of this licentiate thesis was to assess the strengths and limitations of the interrupted time series (ITS) design, which has gained some attention for its ability to provide valid effect estimates. Two national road safety interventions involving VRUs were selected as cases: the Swedish bicycle helmet law for children under the age 15, and the tightening of licensing rules for Class 1 mopeds. The empirical results suggest that both interventions were effective in improving the safety of VRUs. Unless other concurrent events affect the treatment population at the exact time of intervention, the effect estimates should be internally valid. One of the main limitations of the study design is the inability to identify why the interventions were successful, especially if they are complex and multifaceted. A lack of reliable exposure data can also pose a further threat to studies of interventions involving VRUs if the intervention can affect the exposure itself. It may also be difficult to generalize the exact effect estimates to other regions and populations. Future studies should consider the use of the ITS design to enhance the internal validity of before-after measurements.

Abstract [en]

Traffic-related injuries represent a global public health problem, and contribute largely to mortality and years lived with disability. Over the course of the last decades, improvements to road traffic safety and injury surveillance systems have resulted in a shift in focus from motor vehicle accidents to injury events involving vulnerable road users (VRUs), such as cyclists and moped riders. There have been calls for improvements to the evaluation of safety interventions due to methodological problems associated with the most commonly used study designs. The purpose of this licentiate thesis was to assess the strengths and limitations of the interrupted time series (ITS) design, which has gained some attention for its ability to provide valid effect estimates while accounting for secular trends. Two national interventions involving VRUs were selected as cases: the Swedish bicycle helmet law for children under the age 15, and the tightening of licensing rules for Class 1 mopeds. The empirical results suggest that both interventions were effective. These results are discussed in the light of some methodological considerations regarding internal and external validity, data quality and the ability to fully understand key causal mechanisms behind complex interventions. 

Place, publisher, year, edition, pages
Karlstad: Karlstad University Press, 2015. , 80 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2015:22
Keyword [en]
Causal inference, Repeated measures, Methodology, Econometrics, Injury Epidemiology, Vulnerable Road Users, Time series analysis
National Category
Public Health, Global Health, Social Medicine and Epidemiology
Research subject
Risk and Environmental Studies
Identifiers
URN: urn:nbn:se:kau:diva-35781ISBN: 978-91-7063-638-7 (print)OAI: oai:DiVA.org:kau-35781DiVA: diva2:800856
Presentation
2015-05-22, Fryxellsalen, 1B 306, Karlstads universitet, 13:00 (Swedish)
Opponent
Supervisors
Funder
Swedish Civil Contingencies Agency, 2013-4176
Available from: 2015-04-28 Created: 2015-04-08 Last updated: 2015-04-28Bibliographically approved
List of papers
1. The effect of the Swedish bicycle helmet law for children: An interrupted time series study
Open this publication in new window or tab >>The effect of the Swedish bicycle helmet law for children: An interrupted time series study
2014 (English)In: Journal of Safety Research, ISSN 0022-4375, E-ISSN 1879-1247, Vol. 51, 15-22 p.Article in journal (Refereed) Published
Abstract [en]

Previous population-based research has shown that bicycle helmet laws can reduce head injury rates among cyclists. According to deterrence theory, such laws are mainly effective if there is a high likelihood of being apprehended. In this study, we investigated the effect of the Swedish helmet law for children under the age of 15, a population that cannot be fined. Method  An interrupted time series design was used. Monthly inpatient data on injured cyclists from 1998–2012, stratified by age (0–14, 15 +), sex, and injury diagnosis, was obtained from the National Patient Register. The main outcome measure was the proportion of head injury admissions per month. Intervention effect estimates were obtained using generalized autoregressive moving average (GARMA) models. Pre-legislation trend and seasonality was adjusted for, and differences-in-differences estimation was obtained using adults as a non-equivalent control group. Results There was a statistically significant intervention effect among male children, where the proportion of head injuries dropped by 7.8 percentage points. There was no evidence of an intervention effect on the proportion of head injuries among female children. Conclusion According to hospital admission data, the bicycle helmet law appears to have had an effect only on male children.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2014
Keyword
helmet law, cyclist, accident, injury rate, children, patient, swedish
National Category
Public Health, Global Health, Social Medicine and Epidemiology
Research subject
Public Health Science
Identifiers
urn:nbn:se:kau:diva-30119 (URN)10.1016/j.jsr.2014.07.001 (DOI)000345723400003 ()25453172 (PubMedID)
Available from: 2013-11-22 Created: 2013-11-22 Last updated: 2017-12-06Bibliographically approved
2. The effect of stricter licensing on road traffic injury events involving 15 to 17-year-old moped drivers in Sweden: a time series intervention study
Open this publication in new window or tab >>The effect of stricter licensing on road traffic injury events involving 15 to 17-year-old moped drivers in Sweden: a time series intervention study
2015 (English)In: Accident Analysis and Prevention, ISSN 0001-4575, E-ISSN 1879-2057, Vol. 83, 154-161 p.Article in journal (Refereed) Published
Abstract [en]

This study aimed to evaluate and quantify the effect of the introduction of the AM driving license on non-fatal moped-related injuries in Sweden. With the introduction of the new license category in October 2009, prospective moped drivers are now required to pass a mandatory theory test following a practical and theoretical course. In addition, obtaining a license to operate a moped is now considerably more costly.

METHODS:Time series intervention analysis on monthly aggregated injury data (1st Jan 2007-31st Dec 2013) was performed using generalized additive models for location, shape and scale (GAMLSS) to quantify the effect size on injury events involving teenage (15-17 years) moped drivers, while controlling for trend and seasonality. Exposure was adjusted for by using the number of registered mopeds in traffic as a proxy.

RESULTS:The introduction of AM license was associated with a 41% reduction in the rate of injury events involving 15-year-old moped drivers (IRR 0.59 [95% CI: 0.48-0.72]), and a 39% and 36% decrease in those involving 16-year-old (IRR 0.61 [95% CI: 0.48-0.79]) and 17-year-old drivers (IRR 0.64 [95% CI: 0.46-0.90]), respectively. The effect in the 15-year-old stratum was decreased roughly by half after adjusting for exposure, but remained significant, and the corresponding estimates in the other age groups did not change noticeably.

CONCLUSIONS:This study provides quasi-experimental evidence of an effect on non-fatal moped-related injuries as a result of stricter licensing rules. Only part of the effect could be explained by a reduction in the number of mopeds in traffic, indicating that other mechanisms must be studied to fully understand the cause of the reduction in injuries.

Keyword
Injury control; Intervention; Licensing; Policy; Road traffic injuries
National Category
Public Health, Global Health, Social Medicine and Epidemiology
Research subject
Risk and Environmental Studies
Identifiers
urn:nbn:se:kau:diva-34771 (URN)10.1016/j.aap.2015.07.022 (DOI)000362135400015 ()26276538 (PubMedID)
Available from: 2014-12-12 Created: 2014-12-12 Last updated: 2017-12-05Bibliographically approved

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