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BMJ 2005;331:428 (20 August), doi:10.1136/bmj.38537.397512.55 (published 12 July 2005)
PaperRole of mobile phones in motor vehicle crashes resulting in hospital attendance: a case-crossover studySuzanne P McEvoy, senior research fellow1, Mark R Stevenson, professor of injury prevention1, Anne T McCartt, vice president, research2, Mark Woodward, professor of biostatistics1, Claire Haworth, research nurse3, Peter Palamara, senior research officer3, Rina Cercarelli, senior research fellow31 The George Institute for International Health, University of Sydney, PO Box M201, Sydney, NSW 2050, Australia, 2 Insurance Institute for Highway Safety, Arlington, VA, USA, 3 Injury Research Centre, University of Western Australia, Crawley, WA 6009, Australia
Correspondence to: S McEvoy smcevoy{at}thegeorgeinstitute.org
Objectives To explore the effect of drivers' use of mobile (cell) phones on road safety. Design A case-crossover study. Setting Perth, Western Australia.
Participants 456 drivers aged Main outcome measure Driver's use of mobile phone at estimated time of crash and on trips at the same time of day in the week before the crash. Interviews with drivers in hospital and phone company's records of phone use.
Results Driver's use of a mobile phone up to 10 minutes before a crash was associated with a fourfold increased likelihood of crashing (odds ratio 4.1, 95% confidence interval 2.2 to 7.7, P < 0.001). Risk was raised irrespective of whether or not a hands-free device was used (hands-free: 3.8, 1.8 to 8.0, P < 0.001; hand held: 4.9, 1.6 to 15.5, P = 0.003). Increased risk was similar in men and women and in drivers aged Conclusions When drivers use a mobile phone there is an increased likelihood of a crash resulting in injury. Using a hands-free phone is not any safer.
Surveys indicate that drivers often talk on mobile (cell) phones. A 2004 observational survey of drivers of passenger vehicles in the United States indicated that at any given time of day 5% were talking on hand held phones.1 Observational studies in other countries have reported lower rates of use.2-4 Internationally, drivers report usually using hand held phones.5-7 Because of concerns about risks of a potential crash, use of hand held phones is illegal in most countries in the European Union, all Australian states, the Canadian province of Newfoundland and Labrador, and New York, New Jersey, and the District of Columbia in the United States. Most research on the safety implications of drivers' use of mobile phones has been experimental in design, involving small samples of volunteers. These studies have found that phone use impairs performance on simulated or instrumented driving tasks, using such measures as reaction time,8-11 variability of lane position and speed,10 following distance,11 and situational awareness.9 12 Impairments have resulted from cognitive distractions whether drivers are using either hands-free8 9 or hand held13 14 phones. Studies also have reported effects of physical distraction from handling phones.15 16 It is unknown whether experimental findings are applicable to drivers using phones in their own vehicles. A few epidemiological studies have assessed the risk of crashes associated with phone use. Police crash reports are not useful in this regard because information on the driver's phone use is unreliable. Two studies found modest increases in risk among drivers observed using hand held phones17 and among more frequent versus less frequent users according to billing records from mobile phone companies.7 Neither study examined phone use at the time of the crash. Using billing records to verify phone use by drivers involved in crashes that involved damage to property, Redelmeier and Tibshirani18 compared phone use immediately before the crash with use during the previous week. The estimated risk of a crash while using phones was four times higher than when phones were not used. Hands-free phones seemed to offer no safety advantage over hand held phones, though few drivers had hands-free phones. Important questions remain about the effects of phone use on the risk of a crash. These include whether use affects the risk of more serious crashes involving personal injuries and whether the risk differs for hands-free versus hand held phones. We studied drivers involved in injury crashes in Perth, Western Australia, where mobile phone company records could be obtained. Since 1 July 2001 it has been illegal to use a hand held phone when driving in Western Australia.
We used a case-crossover design, a variation of a case-control design that is appropriate when a brief exposure (driver's phone use) causes a transient rise in the risk of a rare outcome (a crash). We compared a driver's use of a mobile phone at the estimated time of a crash with the same driver's use during another suitable time period. Because drivers are their own controls, the design controls for characteristics of the driver that may affect the risk of a crash but do not change over a short period of time. As it is important that risks during control periods and crash trips are similar, we compared phone activity during the hazard interval (time immediately before the crash) with phone activity during control intervals (equivalent times during which participants were driving but did not crash) in the previous week.
Study setting and participants
Data collection The three major telecommunication networks (Optus, Telstra, and Vodafone) provided records of relevant phone activity for consenting participants.
Time of crash
Hazard and control intervals
Statistical analysis
Of 1625 drivers approached, 454 (28%) did not own or use mobile phones, 133 (8%) met another exclusion criterion, and 97 (6%) declined participation. The 941 (58%) remaining drivers were interviewed, and mobile phone activity records of 744 (79%) were available. Reasons for non-availability of records included refusal to allow access (n = 70), use of company phones (n = 47), phone data not accessible (n = 35), other owner (n = 24), and inability to recall phone number (n = 21). Among drivers with available phone records, 456 (61%) verified driving during at least one control interval. These were the study subjects and the basis of the case-crossover analyses. Tables 1 and 2 summarise characteristics of drivers and crashes for drivers who completed interviews, interviewed drivers with available phone records, and the case-crossover drivers.
Most participants (n = 423, 93%) had at least one injury and 44% (n = 201) had two or more. Injuries included sprains and strains (n = 252, 55%), haematomas and bruising (n = 149, 33%), abrasions and lacerations (n = 94, 21%), fractures and dislocations (n = 65, 14%), minor head injuries (n = 35, 8%), internal organ injuries to chest or abdomen (n = 14, 3%), and spinal injuries (n = 8, 2%). Injuries were predominantly mild to moderate in severity. Despite owning or using mobile phones, 28% (n = 126) of participants said they never used the phone while driving. Of those (72%) who reported using a phone while driving, the proportion with hands-free devices ranged from 60% (n = 134) for those who occasionally used a phone while driving to 82% (n = 37) for those who often did. In total, about half the participants (n = 238, 52%) reported having hands-free devices in their vehicles, including earpieces (n = 164, 36%), fully installed hands-free car kits (n = 45, 10%), headsets (n = 20, 4%), and speaker phones on handsets (n = 9, 2%; table 1). Only 6% (n = 29) had phones with voice activation features. Thirty drivers with a hands-free device in their vehicles (13%) said they never used a phone while driving; 159 (67%) reported using the hands-free device for phone activity when driving at least 90% of the time; and only 21 (9%) reported using it less than half the time. Thus, of drivers who had hands-free devices and reported using a phone while driving, almost all said they generally used these devices. Most drivers (n = 411, 90%) were carrying their mobile phones during the crash trips (table 1), and 7% (n = 32) said they used the phone sometime during that trip. About half (n = 234, 51%) reportedly used their phones at least once after crashing, most commonly to contact family members (n = 152, 65%), friends (n = 53, 23%), workplace (n = 42, 18%), and emergency services (n = 31, 13%). Of the 456 participants, 192 (42%) had driven during one control interval, 183 (40%) had driven during two, and 81 (18%) had driven during all three. This resulted in 456 case intervals and 801 control intervals available for analysis with multiple control periods. Phone records indicated that 40 of the 456 subjects (9%) used mobile phones during the hazard interval (that is, up to 10 minutes before the crash). Phones were used during 3% (n = 25) of the 801 multiple control intervals. Based on the reported availability of hands-free devices, about one third (n = 13) of calls during the hazard interval and one third (n = 8) of calls during control intervals were on hand held phones. Of drivers with hands-free devices who used their phones during the hazard interval, 89% (n = 24) reported that they used their hands-free devices for phone activity when driving at least 90% of the time. Mobile phone use within the period during and up to 10 minutes before the estimated time of the crash was associated with a fourfold increase in the likelihood of crashing (odds ratio 4.1, 95% confidence interval 2.2 to 7.7, P < 0.001) (table 3). Similar results were obtained when we analysed only the interval up to 5 minutes before a crash (3.6, 1.8 to 7.0, P < 0.001). Analyses with paired matching to compare the hazard interval with an equivalent single control interval also showed significant associations between mobile phone use and the likelihood of a crash, similar in magnitude to the association with 1:M matching (table 3).
Sex, age group, or type of mobile phone did not affect the association between phone use and risk of crash (P > 0.05) (table 4). In particular, both hand held and hands-free phone use while driving was associated with increased risk (4.9, 1.6 to 15.5, P = 0.003 v 3.8, 1.8 to 8.0, P < 0.001, respectively).
A person using a mobile phone when driving is four times more likely to have a crash that will result in hospital attendance. Sex, age group, or availability of a hands-free device do not influence the increased likelihood of a crash. In this study, we measured the seriousness of crashes by participants' injuries; almost all had at least one injury and almost half had two or more.
Comparison with other research Our findings are similar to those reported by Redelmeier and Tibshirani, whose case-crossover study found a fourfold increased risk of crashes that result in damage to property associated with phone use.18 These similar findings occurred despite some notable differences in the methods of the studies. In our study we collected and reconciled, where possible, both drivers' reports of phone use before crashes and data from phone companies. Because of the higher proportion of drivers with hands-free devices in our study, we were able to estimate the risk of crashes associated with hands-free and hand held phones. We assumed that phone use could have influenced risk of crash only when participants were driving, and participants included only those drivers who reported in their initial interviews that they had been driving during at least one control interval. Redelmeier and Tibshirani did not initially collect information on driving during control intervals.18 They adjusted estimates of risk based on information from a separate pilot study and from interviews with a small subset of participants conducted a year after the crash. Some researchers questioned whether these adjustments adequately addressed the issue.21 22 While our approach did not remove the potential for recall bias, it probably reduced it. The lengths of hazard and corresponding control intervals in our study were not fixed, as used by Redelmeier and Tibshirani,18 but varied up to 10 minutes according to the duration of the driving trip in which a crash occurred. As 63% (n = 288) of drivers reported a trip length of 10 minutes or less before crashing this could have been an important consideration.
Limitations We took care to verify that participants were driving during the hazard and control intervals, but circumstances of the control driving intervals may have differed from the crash driving interval, and the findings point to a statistical rather than causal association. Our results, however, reflect those reported by Redelmeier and Tibshirani,18 as well as those from the numerous experimental studies.8-12 The distracting effects of different types of hands-free phones may not be equivalent—for example, searching for an earpiece to answer an incoming call may be more distracting than answering a phone mounted in a hands-free kit. Although voice activated units are becoming more common, only 6% of mobile phone users in our study had phones with this feature. The sample size was not large enough to assess whether certain types of hands-free devices, including fully hands-free, might be safer than other types.
Policy implications Studies of laws prohibiting use of hand held phones in the US found that such use declined significantly in the first few months after the laws took effect.23 24 In New York, use of hand held phones subsequently returned to levels seen before the law. Publicised enforcement campaigns may be needed to achieve long term compliance. Even full compliance, however, will not eliminate the risk of crashes. According to our study, there is no safety advantage associated with switching to the types of hands-free devices that are commonly in use.
Laws limiting all phone use while driving would be difficult to enforce. While a possible solution in the future is to change mobile phones so they cannot be used when vehicles are in motion, the likelihood the industry would embrace such a change seems remote. More and more new vehicles are being equipped with Bluetooth technology, facilitating voice activation and therefore totally hands-free phone use. Though this may lead to fewer hand held phones used while driving in the future, our research indicates that this may not remove the risk. Importantly, if this new technology actually increases mobile phone use in cars, it could contribute to even more crashes. At least in the short term, it seems likely that mobile phone use in cars will continue to grow, despite the growing evidence of the risk it creates.
We acknowledge the contributions of Margaret Hocking, Peter McEvoy, and Cheri Donaldson. We also thank the participating hospitals (Fremantle, Royal Perth, and Sir Charles Gairdner), St John Ambulance Service, and Optus, Telstra, and Vodafone. Contributors: SPM and MRS contributed to study design, conduct, analysis, and writing of the manuscript. ATM contributed to study design, analysis, and writing of the manuscript. MW contributed to study analysis and writing of the manuscript. CH, PP, and RC contributed to study design and conduct. MRS is guarantor. Funding: Insurance Institute for Highway Safety. SPM received a National Health and Medical Research Council postgraduate public health scholarship. Competing interests: None declared. Ethical approval: Human research ethics committees at the University of Western Australia and the participating hospitals approved the study.
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Abstract
Introduction
17 years who owned or used mobile phones and had been involved in road crashes necessitating hospital attendance between April 2002 and July 2004. 
2 test with one degree of freedom. 
