Background Because of a belief that the use of cellular telephoneswhile driving may cause collisions, several countries have restrictedtheir use in motor vehicles, and others are considering suchregulations. We used an epidemiologic method, the case–crossoverdesign, to study whether using a cellular telephone while drivingincreases the risk of a motor vehicle collision.
Methods We studied 699 drivers who had cellular telephones andwho were involved in motor vehicle collisions resulting in substantialproperty damage but no personal injury. Each person's cellular-telephonecalls on the day of the collision and during the previous weekwere analyzed through the use of detailed billing records.
Results A total of 26,798 cellular-telephone calls were madeduring the 14-month study period. The risk of a collision whenusing a cellular telephone was four times higher than the riskwhen a cellular telephone was not being used (relative risk,4.3; 95 percent confidence interval, 3.0 to 6.5). The relativerisk was similar for drivers who differed in personal characteristicssuch as age and driving experience; calls close to the timeof the collision were particularly hazardous (relative risk,4.8 for calls placed within 5 minutes of the collision, as comparedwith 1.3 for calls placed more than 15 minutes before the collision;P<0.001); and units that allowed the hands to be free (relativerisk, 5.9) offered no safety advantage over hand-held units(relative risk, 3.9; P not significant). Thirty-nine percentof the drivers called emergency services after the collision,suggesting that having a cellular telephone may have had advantagesin the aftermath of an event.
Conclusions The use of cellular telephones in motor vehiclesis associated with a quadrupling of the risk of a collisionduring the brief period of a call. Decisions about regulationof such telephones, however, need to take into account the benefitsof the technology and the role of individual responsibility.
Motor vehicle collisions are a leading cause of death in NorthAmerica; they are the single most frequent cause of death amongchildren and young adults and account for one fatality every10 minutes.1,2,3 During an average year, about 1 person in 50will be involved in a motor vehicle collision; 1 percent ofthem will die, 10 percent will be hospitalized, and 25 percentwill be temporarily disabled.4,5 Motor vehicle collisions ofteninjure persons who are otherwise in good health. The causesof motor vehicle collisions are complicated, but error on thepart of drivers contributes to over 90 percent of events.6
Cellular telephones can be used for placing and receiving telephonecalls while in a motor vehicle. North American sales are enormous;for example, in 1995 the number of new subscribers in the UnitedStates exceeded the birth rate.7,8 Many believe that telephonesmay contribute to collisions by distracting drivers,9 and afew countries (such as Brazil, Israel, and Australia) have lawsagainst using a cellular telephone while driving. Research withsimulators suggests that use of the telephone can impair someaspects of driving performance.10,11,12,13,14 However, industry-sponsoredsurveys have found no increased risk associated with car telephones.15,16
The most rigorous experimental method for testing the effectsof cellular telephones on motor vehicle collisions is to assessoutcomes for persons randomly assigned to use or not use thedevices, but such a study would be very difficult to performand possibly unethical. Instead, we used an epidemiologic method,the case–crossover design, to evaluate potential associationsbetween the use of a cellular telephone and the risk of a motorvehicle collision in real-world circumstances.
Methods
The study was conducted in Toronto, an urban region of 3 millionpeople with no regulations against using a cellular telephonewhile driving. Persons who came to the North York CollisionReporting Centre between July 1, 1994, and August 31, 1995,during peak hours (10 a.m. to 6 p.m.) on Monday through Fridaywere included in the study if they had been in a collision withsubstantial property damage (as judged by the police). Driversdo not report to the center if the collisions involve injury,criminal activity, or the transport of dangerous goods. Driverswere excluded if they said they did not have a cellular telephoneor if their billing records could not be located by May 1, 1996.
Use of Cellular Telephones
Consenting subjects completed a brief questionnaire about theirpersonal characteristics and the features of the collision.We collected telephone records through each person's cellular-telephonenumber and verified each invoice by checking the subject's fullname, mailing address, and calls made to his or her home telephonenumber. For each record, we analyzed all telephone activityon both the day of collision and the preceding seven days, withparticular attention to the time, duration, and direction (incomingor outgoing) of each call. Special note was made of contactwith ambulance personnel, police, or other emergency services.
Time of the Motor Vehicle Collision
The time of each collision was estimated from the subject'sstatement, police records, and telephone listings of calls toemergency services. We classified the times of collisions as"exact" when information from all three sources was availableand consistent or when one source supplied no data but the remainingtwo agreed. Otherwise, we classified the times as "inexact"and used the earliest of the available two or three times toavoid misclassifying calls made after the collision as contributingto the event. Selecting the earliest listed time reduced thechance of finding spurious associations between telephone useand collisions. However, selecting an excessively early timecould lead to the underestimation of the magnitude of any association.
Analytic Method
We used case–crossover analysis, a technique for assessingthe brief change in risk associated with a transient exposure.According to this method, each person serves as his or her owncontrol; confounding due to age, sex, visual acuity, training,personality, driving record, and other fixed characteristicsis thereby eliminated.17 We used the pair-matched analytic approachto contrast a time period on the day of the collision with acomparable period on a day preceding the collision.18 In thisinstance, case–crossover analysis would identify an increasein risk if there were more telephone calls immediately beforethe collision than would be expected solely as a result of chance.
Definitions of Time Periods
We defined the hazard interval to include any telephone callsoccurring during the 10 minutes before the estimated time ofthe collision, and tested the robustness of our results by analyzingintervals of 1, 5, and 15 minutes.19 In the primary analysis,we compared each person's telephone activity immediately beforethe collision (case) to his or her activity during a controlperiod at the same time as the hazard interval on the day beforethe collision (crossover). In supplementary analyses we evaluatedalternative comparison days and considered intervals of an hourleading up to the collision.
Alternative Comparison Days
We checked our estimates by repeating the calculations usingfour other control intervals. In the workday comparison we selectedthe day of the workweek preceding the collision; for example,the period just before a collision on Monday was compared withthe same period on the preceding Friday. In the weekday comparison,we selected the same day one week before the collision; forexample, Monday was compared with the preceding Monday. In thematching-day comparison, we selected the nearest day of thepreceding week on which there was cellular-telephone activityin the predefined lead-up period before the collision. For themaximal-use-day comparison, we used the control interval fromthe preceding three days in which there was the greatest amountof cellular-telephone activity.
Accounting for Intermittency of Driving
Evaluating telephone activity on the day before a collisionis appropriate only if driving occurred during the control intervalon that day. A pilot survey involving 100 subjects indicatedthat 35 percent of them did not drive during the selected period;the rules of conditional probability suggested that this degreeof intermittency of driving would inflate the apparent relationbetween cellular-telephone use and motor vehicle collisionsby a factor of 1.5 (1 ÷ 0.65).20,21 Our estimates ofrelative risk were therefore divided by this factor as one wayof adjusting for the intermittency of driving.
To examine the robustness of our analysis, we also tested adifferent adjustment that relied on individual driving patterns.To do so, between October 25 and November 28, 1996, we attemptedto contact all subjects who had used their cellular telephonesin the 10 minutes before the collision or the 10-minute controlperiod. We asked each person to remember his or her drivingpattern on both the day of the collision and the day beforethe collision. We then recalculated relative risks by limitingthe analysis to subjects who were confident that they had drivena motor vehicle during both periods on both days.
Ethical Issues
The protocol was approved by the University of Toronto HumanEthics Committee, and all participants provided informed consent.Private industry supplied telephone records but otherwise hadno involvement in data collection or analysis or funding thestudy. Individual billing records were obtained directly fromcellular-telephone carriers who provided records for 100 consecutivedays of telephone use for each person and who were not toldwhich particular date was the day of the collision. Police reportswere obtained directly from police departments; they, in turn,were not provided copies of the drivers' cellular-telephonerecords.
Statistical Analysis
The sample size was calculated to provide an 80 percent chanceof detecting a doubling or halving of collision rates. Relativerisks were estimated with methods for matched-pairs studieson the basis of exact binomial tests and conditional logistic-regressionanalyses.22 Confidence intervals for the relative risks werederived with the bootstrap bias-corrected method and accountedfor the uncertainty in the adjustment for intermittency of driving.23,24Modifications of the relative risks were assessed by comparingdifferent subgroups, with particular attention to the prespecifiedcontrast between hand-held cellular telephones and models thatleave the hands free. All P values were two-tailed, and allrelative risks were computed with 95 percent confidence intervals.
Results
We approached 5890 drivers, of whom 1064 acknowledged havinga cellular telephone and 742 consented to participate in thestudy; the billing records of 699 of these drivers were located(Table 1). The collision times were exact for 231 subjects andinexact for 468. The group placed a total of 16,870 cellular-telephonecalls and received 3643 calls during the week before the collisions(average, 3.4 calls placed and 0.7 call received per personeach day). The average duration of the calls was 2.3 minutes,and 76 percent lasted 2 minutes or less (similar to cellular-telephonecalling patterns elsewhere25). The monthly bill in U.S. currencyfor the average participant was $72, which was greater thanthat for the average subscriber in Toronto or the average subscriberin North America ($53 and $51, respectively).26,27,28
Table 1. Characteristics of 699 Drivers and Collisions.
Overall, 170 subjects (24 percent) had used a cellular telephoneduring the 10-minute period immediately before the collision,37 (5 percent) had used the telephone during the same periodon the day before the collision, and 13 (2 percent) had usedthe telephone during both periods. The crude analysis indicatedthat cellular-telephone activity was associated with a relativerisk of a motor vehicle collision of 6.5 (95 percent confidenceinterval, 4.5 to 9.9). The primary analysis, adjusted for intermittentdriving, indicated that cellular-telephone activity was associatedwith a quadrupling of the risk of a motor vehicle collision(relative risk, 4.3; 95 percent confidence interval, 3.0 to6.5).
At follow-up in 1996, we located 145 subjects, of whom 72 (50percent) were confident that they had driven during both thehazard period and the control period. Restricting our analysisto this subgroup yielded an estimated relative risk of 7.0 (95percent confidence interval, 3.7 to 15.5) associated with cellular-telephoneuse. An analysis that included the entire cohort of 699 driversand used alternative comparison days yielded similar estimatesof the relative risk of a collision (Figure 1). All the alternativeestimates of relative risk were adjusted for intermittent driving,and all were statistically significant (P<0.001).
Figure 1. Relative Risk of a Collision for Different Control Periods.
Relative risks were calculated for five different control intervals. In the day-before comparison, we used the control period on the day immediately before the collision; in the workday comparison, the period on the preceding day of the workweek; in the weekday comparison, the period on the day one week before the collision; in the maximal-use-day comparison, the day with the most cellular-telephone activity of the three days preceding the collision; and in the matching-day comparison, the period on the nearest day of the preceding week in which there was cellular-telephone activity in the lead-up period. The vertical lines indicate 95 percent confidence intervals. Bars entirely above 1 indicate statistically significant associations (P<0.05).
The relative risk of a collision associated with using a cellulartelephone was consistent among subgroups with different characteristics(Table 2). Younger drivers were at a somewhat higher relativerisk when using a cellular telephone than older drivers, althoughthe trend was not significant. In no group did cellular-telephoneuse have a protective effect. In particular, subjects with manyyears of experience in using a cellular telephone still hada significant increase in risk. The highest relative risk wasfound among subjects who had not graduated from high school.Telephones that allowed the hands to be free did not appearto be safer than hand-held telephones.
Table 2. Relative Risk of a Motor Vehicle Collision in 10-Minute Periods, According to Selected Characteristics.
The increase in risk appeared to be greatest for calls madenear the time of the collision, and was not statistically significantfor calls made more than 15 minutes before the event (Figure 2).The relative risk was 4.8 for calls within 5 minutes beforethe collision, as compared with 1.3 for calls more than 15 minutesbefore the collision (P<0.001). The risks were similar atdifferent times of the day and of the week (Figure 3). Estimatesappeared robust when calculated with use of hazard intervalsof 1, 5, or 15 minutes before the collision (relative risks,4.7, 4.8, and 4.3, respectively), for data including exact ratherthan inexact times of collisions (4.0 and 4.5, respectively),and with only incoming calls or only outgoing calls included(3.0 and 3.8, respectively). The association appeared strongerfor collisions on high-speed roadways than for collisions inparking lots, at gas stations, or in other low-speed locations(5.4 vs. 1.6, P = 0.014).
Figure 2. Time of Cellular-Telephone Call in Relation to the Relative Risk of a Collision.
Each minute before the collision was assessed as an independent hazard interval, with these intervals grouped in five-minute periods. Cellular-telephone activity for each hazard interval was evaluated in relation to the same period on the day before the collision. Relative risks greater than 1 indicate an association between telephone use and collisions. The vertical lines indicate 95 percent confidence intervals. Bars entirely above 1 indicate statistically significant associations (P<0.05). Calls made 1 to 5 minutes before the collision were significantly riskier than calls made 16 to 20 minutes before the collision (P<0.001).
Figure 3. Consistency of Relative Risks Obtained from Different Collision Times.
The graph shows estimates of relative risk for collisions at different times of the day and of the week. Morning was defined as 8 a.m. to 11:59 a.m., afternoon as noon to 3:59 p.m., evening as 4 p.m. to 7:59 p.m., and other as all remaining times. Saturday and Sunday are combined in a single weekend category. The vertical lines indicate 95 percent confidence intervals. Bars entirely above 1 indicate statistically significant associations (P<0.05). The vertical scale is logarithmic.
A total of 5325 calls were placed and 960 calls were receivedon the collision days, of which the majority occurred afterthe event (68 percent and 64 percent, respectively). About 39percent of the subjects used their cellular telephone at leastonce to contact emergency services immediately after the collision.The median number of calls made during the remainder of theday after the collision was substantially greater than the mediannumber of calls made during an entire day before the collision(four vs. two, P<0.001). Of those who had not used theirtelephone on any day before the collision, 14 of 39 (36 percent)made at least one call in the aftermath of the event.
Discussion
We found that using a cellular telephone was associated witha risk of having a motor vehicle collision that was about fourtimes as high as that among the same drivers when they werenot using their cellular telephones. This relative risk is similarto the hazard associated with driving with a blood alcohol levelat the legal limit.29,30,31 We also found that cellular telephoneshave benefits, such as allowing drivers to make emergency callsquickly. A few drivers used their telephones only in the aftermathof a collision, thereby gaining some potential benefits andincurring no potential risks due to telephone use. In general,cellular-telephone calls were brief and infrequent, which explainswhy the rapid growth of this technology during recent yearshas not been accompanied by a dramatic increase in motor vehiclecollisions.32
We observed no safety advantage to hands-free as compared withhand-held telephones. This finding was not explained by imbalancesin the subjects' age, education, socioeconomic status, or otherdemographic characteristics. Nor can it be explained by suggestingthat those with units that leave the hands free do more driving.One possibility is that motor vehicle collisions result froma driver's limitations with regard to attention rather thandexterity.33 Regardless of the explanation, our data do notsupport the policy followed in some countries of restrictinghand-held cellular telephones but not those that leave the handsfree.
Three weaknesses of this study should be pointed out. First,we studied only drivers who consented to participate. The factthat some persons chose not to consent might have caused usto underestimate the risks associated with telephone use ifthese people declined because of concern about personal liability.Second, people vary in their driving behavior from day to day— a fact that makes the selection of a control periodproblematic. However, it would be difficult to explain all ourfindings on the basis of different driving patterns, and inparticular, this factor would not account for the similar resultsfor those who remembered driving during both periods on bothdays. Third, case–crossover analysis does not eliminateall forms of confounding. Imbalances in some temporary conditionsrelated to the driver, the vehicle, or the environment are possible,but we believe such factors are not likely to account for themagnitude of the association we observed.
Our study indicates an association but not necessarily a causalrelation between the use of cellular telephones while drivingand a subsequent motor vehicle collision. For example, emotionalstress may lead to both increased use of a cellular telephoneand decreased driving ability. If so, individual calls may donothing to alter the chances of a collision. In addition, ourstudy did not include serious injuries; hence, we do not knowhow — or whether — cellular-telephone use is associatedwith motor vehicle fatalities. Finally, the data do not indicatethat the drivers were at fault in the collisions; it may bethat cellular telephones merely decrease a driver's abilityto avoid a collision caused by someone else.
We caution against interpreting our data as showing that cellulartelephones are harmful and that their use should be restricted.Even if a causal relation with motor vehicle collisions wereto be established, drivers are vulnerable to other distractionsthat could offset the potential reductions in risk due to restrictingthe use of cellular telephones. Regulations would also meanreducing benefits; in Canada, for example, half a million callsto 911 emergency services are made from cellular telephoneseach year.34 Yet proposals for regulation are not unreasonable,since poor driving imposes risks on others. Public debate isneeded, given that cellular telephones contribute to improvementsin productivity, the quality of life, and peace of mind formore than 30 million people in North America alone.
The role of regulation is controversial, but the role of individualresponsibility is clear. Drivers who use a cellular telephoneare at increased risk for a motor vehicle collision and shouldconsider road-safety precautions. For them as for all otherdrivers, these include abstaining from alcohol, avoiding excessivespeed, and minimizing other distractions. Additional strategiesmight include refraining from placing or receiving unnecessarycalls, interrupting telephone conversations if necessary, andkeeping calls brief — particularly in hazardous drivingsituations. Physicians should also learn to recognize patientswho are at risk for a collision and who may benefit from adviceregarding safety.35,36,37,38,39,40 Even limited success in reducingrisk may prevent some of the death, disability, and propertydamage related to motor vehicle collisions.
Supported by a Career Scientist Award from the Ontario Ministryof Health (to Dr. Redelmeier), a grant from the Natural Sciencesand Engineering Research Council of Canada (to Dr. Tibshirani),and a grant from the Ontario Ministry of Transportation.
We are indebted to Bell Mobility, Inc., Rogers Cantel, Inc.,and the Metropolitan Toronto Police Department for their cooperation;and to the following people for help with specific aspects ofthe study: Mary Chipman, Ruth Croxford, Allan Detsky, BradleyEfron, Trevor Hastie, Mark Henkelman, Philip Lavori, Barry McLellan,David Naylor, Simon Min, Miriam Shuchman, and Amos Tversky.
Source Information
From the Departments of Medicine (D.A.R.), Preventive Medicine and Biostatistics (R.J.T.), and Statistics (R.J.T.) and the Program in Clinical Epidemiology and Health Care Research (D.A.R.), University of Toronto, Toronto; and the Division of Clinical Epidemiology, Sunnybrook Health Science Centre, North York, Ont., Canada (D.A.R.).
Address reprint requests to Dr. Redelmeier at Sunnybrook Health Science Centre, G-151, 2075 Bayview Ave., North York, ON M4N 3M5, Canada.
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Cellular Telephones and Traffic Accidents
Cohen P. J., Quinlan K. P., Paltiel O., Ambrose A., Redelmeier D. A., Tibshirani R. J., Maclure M., Mittleman M. A.
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N Engl J Med 1997;
337:127-129, Jul 10, 1997.
Correspondence
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