A statistical model to compare road mortality in OECD countries

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Abstract

The objective of this paper is to compare safety levels and trends in OECD countries from 1980 to 1994 with the help of a statistical model and to launch international discussion and further research about international comparisons. Between 1980 and 1994, the annual number of fatalities decreased drastically in all the selected countries except Japan (+12%), Greece (+56%) and ex-East Germany (+50%). The highest decreases were observed in ex-West Germany (−48%), Switzerland (−44%), Australia (−40%), and UK (−39%). In France, the decrease in fatalities over the same period reached 34%. The fatality rate, an indicator of risk, decreased in the selected countries from 1980 to 1994 except in the east-European countries during the motorization boom in the late 1980s. As fatality rates are not sufficient for international comparisons, a statistical multiple regression model is set up to compare road safety levels in 21 OECD countries over 15 years. Data were collected from IRTAD1 (International Road Traffic and Accident Database) and other OECD statistical sources. The number of fatalities is explained by seven exogenous (to road safety) variables. The model, pooling cross-sectional and time series data, supplies estimates of elasticity to the fatalities for each variable: 0.96 for the population; 0.28 for the vehicle fleet per capita; −0.16 for the percentage of buses and coaches in the motorised vehicle fleet; 0.83 for the percentage of youngsters in the population; −0.41 for the percentage of urban population; 0.39 for alcohol consumption per capita; and 0.39 for the percentage of employed people. The model also supplies a rough estimate of the safety performance of a country: the regression residuals are supposed to contain the effects of essentially endogenous and unobserved variables, independent to the exogenous variables. These endogenous variables are safety performance variables (safety actions, traffic safety policy, network improvements and social acceptance). A new indicator, better than the mortality rate, is then set upon the residuals. Mean estimates of this indicator for the years 1980–1982 and the years 1992–1994 rank the countries in the beginning and at the end of the study period. Countries showing the best ranks (and thus the best performance) in 1980 and 1994 are Sweden, the Netherlands and Norway. The UK and Switzerland reach the top 5 in 1994. Greece, Belgium, Portugal and Spain are the last countries in the classification along with, surprisingly, the USA. France was ranked 18th in 1980 and 15th in 1994 but is ranked amongst the five countries that most improved from 1980 to 1994. This model remains non definitive because it is not able to distinguish between safety performance and unobserved exogenous variables although these exogenous variables could explain more about the differences in levels and trends between the countries. More complex models, particularly highly sophisticated models regarding the number of fatalities with breakdowns by road users or road classes would be needed to give a precise and profound ranking of safety levels and safety improvements between countries.

Introduction

Life and death depend on destiny. (Confucius)…… But, as far as road safety is concerned, they do not. Our planet shelters about 6 billion people, more than 22 million km of roads, 470 million passenger cars and 145 million station wagons, vans and trucks2. One third of motorised vehicles move in the USA and another third in the European Union (EU) (ONISR, 1996, International Road Federation, 1995). According to the World Health Organization, 600 000 people die and 15 million are injured every year in traffic accidents3. Two thirds of the casualties occurred in developing countries. Seventy-five percent of casualties in these countries are vulnerable road users such as pedestrians or two-wheelers (Downing, 1995).

The EU counts about 368 million inhabitants and 183 million motorised vehicles. A total of 1 200 000 injury road accidents and 45 700 fatalities occur every year, slightly more than in the USA (42 300 in 1995) with 260 million inhabitants and 198 million motorised vehicles.

In most countries, accident economic losses reach 1 or 2% of GNP (Ross and Ray, 1995). In 1997, the European Transport Safety Council estimated the total cost of transport accidents in Europe at 166 billion Euros (ETSC, 1997). Ninety-seven percent of these costs, i.e. 162 billion Euros, were directly related to road transport.

Road safety has been increasing in industrialised countries for 25 years and this increase shows that political willingness and counter-measures produce results (Broughton, 1991, Downing, 1995). Improvements in road safety and the numerous results in the evaluation of safety measures led some countries to compare their performance and to use these comparisons as a tool to evaluate the effectiveness of their safety policy. It is then rightful to wonder whether the common indicators (fatalities and fatalities rates) are adapted to evaluation.

Numerous factors influence a country's safety level. These factors are concerned with road safety policy, distribution and crashworthiness of the fleet, road network characteristics, human behaviour and attitudes, and so on (Brenac, 1989, Brühning, 1995, Sivak, 1996). Among these factors, some are exogenous to road safety pure performance (safety actions, traffic safety policy and social acceptance), and some are endogenous (Poppe, 1995).

The evaluation of safety actions by international comparison consists in evaluating the performance of a country, i.e. the effectiveness of endogenous factors in each country, the exogenous factors being neutralised. The performance must be understood as the ability of a road safety policy to be effective and the ability of a population to accept and respect this policy. This performance measures the pure success of a country in its struggle against road mortality, independently of exogenous factors that influence the road safety level but which can not be modified by safety policy.

For example, the age pyramid is exogenous to safety performance: in all countries, the accidental risk is at least three times higher for youngsters than for older people. Therefore, it is incontestable that a young country would count, ceteris paribus, more people killed on the roads than an old country. On the other hand, the number of roundabouts is supposed to be endogenous to road safety policy since this feature comes from a strategic arbitration between several features able to improve the safety of intersections.

It is obvious that international comparison of fatality rates alone is not satisfactory to evaluate performance since the calculation of the rates refers to a single denominator at the same time, which plays the role of the exogenous factor (vehicle fleet, kilometrage, population). This unique factor becomes the standardisation. It is a real step forward compared with the absolute number of fatalities, but remains insufficient because other important exogenous factors are not taken into account. And anyway, the fatality rate is either not available, or inaccurate or inconsistent since the estimation methods of kilometrage differ between countries.

If it is not entirely right to compare countries by using fatality rates, it is frequent to propose comparisons by using their progression which better reveals the safety improvements assuming constant exogenous factors over time. Unfortunately, in some cases, this assumption can not hold true. In other cases time series comparisons are dependant on the initial level of kilometrage and on the origin and end periods of comparisons (Andreassen, 1991).

It is therefore necessary to build more relevant or less defective indicators, which is the main objective of this paper (Section 4). This job required a statistical model that combines annual cross-sectional and time series data in 21 OECD countries from 1980 up to 1994.

In Section 2, I present some of the international data available in the International Road Traffic and Accident Database (IRTAD) and then recall, in the second section, the main lessons to be kept in mind from a literature survey in selecting the methods to be used to determine the new indicators of international comparisons. Doing so, the method will also reveal a property: the quantification of the effects of some exogenous factors on the fatalities.

All the factors have not been identified and the systematic statistical representations4 of these factors were also not found in some cases.

Section snippets

International data

The IRTAD (the OECD database) is one of the more convenient data providers for administration or research purposes since it offers a large range of time series data with regards to area of state, population with breakdowns by age, vehicle fleet (moped and mofas, motorcycles, personal cars and station wagons, trucks, buses and coaches), network length (urban, rural, motorways, A-level roads), kilometrage (urban, rural, motorways, A-level roads), kilometrage with breakdown by road users, injury

Epidemiological aspects of international comparisons

Descriptive statistics from IRTAD showed the great variability of safety and traffic indicators values between countries and over time. Descriptive epidemiology usually holds a more reliable mortality indicator than fatality rate: the age-standardised mortality rate which is curiously missing in the international reports or databases except in WHO's. The standardisation corrects the mortality by accounting for the age pyramid. If youngsters present a high risk all over the world, a young

An indicator for international comparisons

Looking at this review, the number of fatalities was modelled macroscopically with a pooling method combining time series and cross sections.

Discussion and conclusion

In Section 2 of this paper, I presented some of the international sources and data suppliers of road safety statistics. About ten institutions release international reports and/or supply databases about road safety. Statistics are usually available in the n+2 year except for general statistics such as injury accidents or fatalities available sometimes very quickly for immediate analysis (European Commission, 1994, OECD, 1997). Some of the statistics are released with a breakdown by road

Acknowledgements

This paper was written in 1997 while I was working for the French Road Safety Department (RSD). I wish to thank Olivier Noël (RSD) and all IRTAD Operational Committee members who encouraged me to carry out this international comparison about road mortality, especially Frank Poppe (NL), Peter Holló (H), Göran Nilsson (S) and Ekkehard Brühning (D). I am particularly indebted on Sylvain Lassarre (F) for his very precious advice and to two anonymous referees for their helpful suggestions. However,

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