The effects of studded tires on fatal crashes with passenger cars and the benefits of electronic stability control (ESC) in Swedish winter driving

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Abstract

This study set out to examine the effects of studded tires on fatal crashes on roads covered with ice or snow in Sweden and also to investigate the extra benefits of electronic stability control (ESC) during the winter months. Two different studies are presented in this paper. Both studies used an induced exposure approach. In the main study, 369 in-depth studies of fatal crashes with passenger cars were analyzed to determine whether loss-of-control (LOC) had been a major component or not. Only crashes involving cars without ESC and equipped with approved studded or non-studded winter tires were analyzed. The additional study used police-reported crashes that occurred during the winter seasons 2003–2010, involving passenger cars with and without ESC. While police records in Sweden do not include any tire information, it was assumed that most cars involved in crashes during the winter period would be equipped with studded tires.

Findings in the main study showed that in 64% of the fatal crashes on roads covered with ice or snow LOC had been a major component. Furthermore, in 82% of LOC crashes, the passenger car over-steered prior to collision. Studded tires were found to have a statistically significant effect of 42% in terms of fatal crash reduction on roads covered with ice or snow, compared to non-studded winter tires. The effect on dry or wet roads in the winter was negative, although statistically non-significant. In the additional study, it was found that ESC further reduced crashes with injuries by 29%. The benefits on severe and fatal crashes were slightly greater (32%), although the lower 95% confidence limit was lower.

Although studded tires were shown to reduce the risk of fatal crash involvement, compared to non-studded winter tires, the proportion of LOC and over-steering among cars with studded tires was large (59% and 49%, respectively). It was therefore concluded that studded tires do not prevent all LOC crashes, while ESC has benefits in those crashes since this technology mostly addresses over-steering. This is also supported by the fact that the share of LOC fatal crashes is considerably lower for ESC-equipped cars.

This study recommends that non-ESC cars should be fitted with studded tires if they are to be driven on roads covered by ice or snow. If the proportion of studded tires is to be decreased on Swedish roads to reduce the about of hazardous particulates especially in built up areas, from a road safety point of view it is recommended that this should be done in phase with the implementation of ESC on all passenger cars.

Introduction

The importance of tires for road safety has been studied over a long period of time. In particular, the importance of winter tires and the influence of studs on winter tires have been subjected to many studies Elvik (1999). This is of extra interest in the some geographical areas, i.e. Sweden, where long and cold winters result in a large proportion of traffic and crashes on roads covered with ice or snow. Approximately 20% of all fatal crashes with passenger cars occur on snow and ice throughout the year in Sweden, while during the winter period the share is about 50%. Naturally, this share varies from year to year (Fig. 1) and between the different geographical regions (Fig. 2).

The effectiveness of studded versus non-studded tires became an issue already in the early seventies, showing large effects on the risk of crashes under icy or snowy road conditions. In a meta-analysis Elvik (1999) however found, that a more comprehensive evaluation shows a limited effect of studded tires, in relation to non-studded winter tires. The reduction of crashes was found to be 5% and non significant for roads covered with ice or snow. On dry or wet roads the effects of studded tires was 2%, also non-significant. More recent studies and calculations have been done in response to problems with air quality related to studded tires. In these studies and calculations, Gustafsson et al. (2006) estimated the typical added safety effect of studded tires compared to non-studded winter tires to 0–10% reduction of passenger car crashes over a winter period and 25% crash reduction on roads covered with ice and snow. Studies from Norway have though shown that there is no significant increase of crashes as a result of reduced fitment of studded tires. However, these studies analyzed the number of crashes inside urban areas before and after the reduction of studded tire fitment, and not on the overall effects, even in rural areas.

In general these studies however have not taken crash type or injury severity into account. Findings of added safety effects in different studies could also vary due to different study objectives. The effect in crash reduction of studded tires will vary greatly if it is calculated throughout the entire year, the winter period, roads cover with ice or snow, a specific crash type or on different injury severities (Elvik, 1999, Gustafsson et al., 2006).

The effects of thread depth of tires have also been studied. Elvik concluded from a number of studies that there is a typical negative effect for thread depth less than 5 mm, while there seems to be no added positive effect over 5 mm (Transportøkonomisk institutt, 2007).

The main problems associated with effectiveness studies of studded tires, or any type of tire, are the possible or likely effects of confounders like other risks factors correlated with the choice of tires, if behavioral modification should be included or not, and the classification of crashes. Types of tires are probably not chosen randomly over the population, but rather on a needs basis as well as related to economy that might introduce selective recruitment.

Several studies have demonstrated a behavioral adaptation to the type of tire, in that drivers of cars with studded tires drive faster than drivers of cars with other tires. While it is not clear if this is a consequence of the improved road grip or that drivers choosing a higher driving speed tend to use studded tires more frequently. Mäkinen (1996) showed that the improved grip from studded tires lead to higher speed on roads with low friction. In that case it would be natural to include the adaptation in the analysis of the effectiveness of studded tires.

Another factor that should be taken into account is that there may be long periods of time during the winter with wet and dry road surface. According to the Scandinavian Tire and Rim Organization (STRO), there are mainly two types of non-studded winter tires on the Swedish market. One suited for Nordic conditions and one for Central European conditions. While Nordic non-studded winter tires are mostly designed for roads covered with ice and snow, Central European non-studded winter tires are designed for milder conditions and higher speeds (STRO, 2010). On wet or dry roads non-studded winter tires might perform better, with regards to stopping distances and stability. This has also been shown in tire tests performed by automobile journals, Auto Motor and Sport (2008).

The problem of crash classification is also a reason for varying and not consistent estimates of the effectiveness of studded tires. It is quite clear, that studded tires primarily affect a certain type and amount of crashes, like loss-of-control (LOC) in certain weather or road conditions. Such a classification of crashes is, however, rare and methods based on induced exposure are therefore complex to use. Types of tires classification in mass data sets are also not common.

The type of vehicle is also a confounding factor. Forward or rearward drive as well as the presence or absence of ESC might or is known to have a major impact on the risk of certain crash types. In particular, ESC has been shown to have major benefits on wet or icy roads. Lie et al. (2006) have shown a minimum of 35% effectiveness for single/oncoming/overtaking serious and fatal crashes. However, the interaction of ESC and types of tires has not been found in the literature.

Studded tires have been a raising issue in Sweden during the last years. Due to their significant contribution to airborne particles (Gustafsson et al., 2006) and the failure in several larger cities to meet the European Union legislation, some municipalities have been given the possibility to ban studded tires on some streets were the air quality is particularly poor. These issues motivated a reduction of studded tires in these areas. However, questions are raised about the road safety consequences of this legislation. Hence, there is a need to find the importance for choice of tires for passenger cars, both with and without ESC, in order to give consumers a balanced recommendation taking safety and the environmental effects into account.

This project was carried out in two separate studies. The objectives in the main study were to use in-depth data and:

  • analyze the crash dynamics in fatal crashes with passenger cars during the winter period,

  • investigate the effect of studded tires on fatal crashes with passenger cars in different winter road conditions, compared to non-studded winter tires,

  • investigate the influence of tire characteristics (e.g. emplacement of best tires, thread depth, condition) on loss-of-control (LOC) crashes,

  • discuss possible confounders not possible to quantify in the analysis.

The objectives in the additional study were to use police-reported mass data and:

  • estimate any benefit provided by ESC in reducing crashes during the winter period when many cars are equipped with winter tires.

Section snippets

Methods

As mentioned above the project was carried out in two separate studies. The main study investigated the effect of studded tires on fatal crashes on different winter road conditions. An additional study was also carried out in order to investigate benefits of ESC on a passenger car population which was reasonably assumed to have a large proportion of studded tires. Fig. 3 reassumes how the main and additional studies interact.

Both studies used an induced exposure approach. As mentioned above,

Main study – the effect of studded tires

The Swedish Transport Administration (STA) has been carrying out in-depth studies for each fatal road crash since 1997. Crash investigators at STA systematically inspect the vehicles involved in fatal crashes and record direction of impact, vehicular intrusion, seat belt use, airbag deployment, tire properties, etc. The crash site is also inspected to investigate road characteristics, collision objects, etc. Further information about injuries is provided by forensic examinations, questioning

Main study – the effect of studded tires

As shown in Table 2, analysis of the in-depth studies showed that 64% of fatal crashes with passenger cars without ESC on roads covered with ice or snow involved LOC. Loss-of-control crashes were relatively less frequent on dry or wet asphalt (18%). It was also found that, when LOC occurred, the distribution of crashes involving over-steering ranged from 76% to 100%.

The analyzed material showed the same distribution of crash type as the national statistics for fatal crashes with passenger cars

Discussion

In the main study, studded tires on cars without ESC were found to have a statistically significant crash reduction of 42% on road covered with ice or snow, compared to non-studded winter tires. As mentioned above, these road conditions may occur with different frequency depending on geographical region. In urban areas in southern Sweden only just over 10% of the fatal crashes occur on snow or ice while in rural areas in northern Sweden the same proportion is approximately 90%. Clearly, this

Conclusions

There were a number of important major findings in this research:

  • 64% of fatal crashes with passenger cars without ESC on roads covered with ice or snow involved LOC. Loss-of-control crashes were relatively less frequent on dry or wet asphalt (18%).

  • When LOC occurred, the distribution of crashes involving over-steering ranged from 76% to 100%.

  • Studded tires were found to reduce fatal crashes with passenger cars on roads covered with ice and snow with 42% compared to non-studded winter tires. The

Recommendations

  • If a car is not intended to be used on roads covered with ice and snow in the winter, the type of winter tires does not seem be of any greater importance. Especially in the mid and southern part of Sweden.

  • If a non-ESC car is intended to be used frequently at any time and in any conditions during the winter, it should be fitted with studded tires.

  • If a car is equipped with ESC, it is likely that the type of winter tires is not crucial to prevent LOC crashes. However, further research is needed.

  • If

Acknowledgements

This study has been funded by the Swedish Transport Administration (STA). Many thanks to Torsten Johansson (STRO) and Pontus Grönvall (Swedish Council for tire information) for valuable support in providing information on non-studded winter tires. Many thanks also to Emely Knudsen at Vectura Consulting. To Therese Höök, Östen Johansson and Martin Juneholm at STA for valuable input and to Simon Sternlund for editorial assistance.

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