Brief Report: The Association of Graduated Driver Licensing with Nondriver Transport-related Injuries Among Adolescents.

BACKGROUND: As a phased approach to initiating driving, graduated driver licensing restricts driving by young drivers with the aim of reducing crashes. It might increase riding with parents or on buses, which might be safer, or walking or biking, which might be more dangerous. We examined whether it increases nondriver injuries, and whether it reduces total injuries combining drivers and nondrivers.
METHODS: We conducted longitudinal analyses of 1995-2012 traffic injuries from 43 states. Using Poisson mixed regression, we estimated adjusted rate ratios for visible, incapacitating, and fatal injury.
RESULTS: Among 16 year olds, graduated driver licensing was associated with reduced passenger injuries (adjusted rate ratio 0.93, 95% confidence interval: 0.89, 0.97). It was not associated with increased injuries as bus riders, pedestrians, or bicyclists among 16- or 17-year olds. It was associated with a 10% reduction in total injuries among 16-year olds, but not 17-year olds.
CONCLUSION: Graduated driver licensing was associated with reduced passenger injuries and total injuries.

Traffic crashes, the leading cause of teenager deaths in the US, accounted for 26% (2,163) of 2013 deaths among 16- to 19-year olds.[1] Florida implemented graduated driver licensing in 1996, and by January 2012, all states and the District of Columbia had implemented some form of graduated driver licensing.[2,3] It typically requires drivers under 18 to proceed through phases: extended learner permit (supervised driving for 3 to 12 months); intermediate (unsupervised driving in daylight, but restricted nighttime driving and, in many states, limited passengers); and full licensure (unlimited unsupervised driving).[4]

State-[3,5-13] and national-level[14-20] studies have reported 10%–40% reductions in crash rates for 16-year-old drivers after the implementation of graduated driver licensing. However, few have considered injury rates of 16- and 17-year-old passengers, bicyclists, pedestrians, and bus riders, as a result of graduated driver licensing.[10,18,21] Restricted driving may increase use of alternative transportation. Riding with parents or on buses may be safer;[22,23] however, walking and biking may be more dangerous.[24] Previous national studies used fatalities;[14-20] however, injuries, more common than fatalities, produce a sample size adequate to detect moderate associations. We conducted longitudinal analyses of traffic injuries in 43 states to estimate the association of graduated driver licensing with injuries among 16- and 17-year-old passengers, bus riders, bicyclists, and pedestrians; and with total injuries (including in drivers).

METHODS

Counts of Visible Injuries in Crashes

We used crash data from 1995 to 2012 from 43 states to obtain counts of injuries (eTable, http://links.lww.com/EDE/B55, which presents crash data year and graduated driver licensing status by state). Data were obtained from police accident reports from National Highway Traffic Safety Administration’s State Data System[25] and from states. Data from the remaining seven states were not available because of no response, cost to acquire, and data format/quality issues. Police accident reports record crashes involving at least one motor vehicle traveling on a road customarily open to the public that result in injury or major property damage.[25] To ensure consistent reporting across states, we included visible, incapacitating, and fatal injuries, and excluded possible injuries and property-damage-only crashes.

Classifying Graduated Driver Licensing

The presence of graduate driver licensing was determined for systems with a learner permit phase ≥3 months, plus an intermediate phase restriction on nighttime driving or number of young passengers.[2]

Population Estimates

Midyear population estimates by state, age, and year were obtained from the US Census Bureau; quarterly values were linearly interpolated.[26]

Other Traffic Laws

The presence of other traffic safety laws may confound the relationship between graduated driver licensing and outcomes. We obtained states’ impaired driving laws (minimum blood alcohol concentration, zero tolerance, and mandatory suspension for driving under the influence) and maximum speed limits.

Economic Indicators

We obtained state-specific quarterly per capita income estimates from the US Bureau of Economic Analysis.[27] The national annual consumer price index was used to adjust for inflation.[28] Each state’s annual unleaded gasoline price was obtained from the Energy Information Administration,[29] then adjusted for inflation.

Statistical Analysis

We estimated per person-year injury rates for passengers, bus riders, bicyclists, pedestrians, and drivers by dividing counts of visible, incapacitating, and fatal injuries by population estimates.

The adjusted rate ratio (aRR) for injury under graduated driver licensing compared with no graduated driver licensing was estimated using Poisson mixed regression. The outcome was the quarterly count of injuries with an offset equal to the log of the population for that quarter. Random intercepts were used to account for the repeated measures from each state over the study period, and robust variance estimation was employed to account for potential model misspecification (e.g., overdispersion).[30] Regression models included terms for year (linear spline terms with knots at 2002 and 2005), quarter (three indicator variables for four quarters), age (0 for 16, 1 for 17), and graduated driver licensing (0 for absence, and 1 for presence), as well as an interaction for age and graduated driver licensing to examine age variation. We further adjusted for traffic laws and state economic factors. In sensitivity analysis of model choice, we examined whether inclusion of random time trends for each state would change estimates for drivers and passengers. The study was approved by West Virginia University’s IRB.

RESULTS

There were 2,400 state quarters from 43 states; graduated driver licensing was absent for 37% of quarters. Two states (Indiana and Wyoming) did not adopt graduated driver licensing during the study period (eTable, http://links.lww.com/EDE/B55, which presents crash data year and graduated driver licensing status by state). Among 16- to 17-year olds, 383,724 passengers, 3,526 bus riders, 24,808 bicyclists, 35,003 pedestrians, and 565,602 drivers were injured from crashes (Table). Graduated driver licensing was associated with reduced total injuries for age 16 (aRR: 0.90, 95% confidence interval [CI]: 0.87, 0.93), but not age 17 (aRR: 1.03, 95% CI: 0.98, 1.08). It was associated with reduced injury rates for 16-year-old but not for 17-year-old drivers (age 16: aRR: 0.87, 95% CI: 0.83, 0.92; age 17: aRR: 1.05, 95% CI: 0.98, 1.12). It was associated with reduced injury rates for 16-year-old passengers (aRR: 0.93, 95% CI: 0.89, 0.97) but not 17-year olds (aRR: 1.02, 95% CI: 0.97, 1.07). The associations between graduated driver licensing and bus riders, bicyclists, and pedestrians were all near null (Table). There was a distribution of state-specific estimates of association between graduated driver licensing and bicyclist, pedestrian, and bus rider injuries. This distribution was centered on the null, and no geographic region or state characteristic was particularly common among states at the low or high end of the distribution. The inclusion of random time trends for each state in the model did not change the aRR for passengers. The aRR was 0.85 (95% CI: 0.81, 0.89) and 1.02 (95% CI: 0.96, 1.09) for 16- and 17-year-old drivers, similar to the main analysis.

DISCUSSION

Graduated driver licensing was associated with reduced driver and passenger injuries for adolescents particularly those aged 16 years. A New York study reported the rate ratio of fatal and incapacitating passenger injuries before and after graduated driver licensing was 1.19 (0.77, 1.84) for age 16 and 0.93 (0.61, 1.43) for 17;[10] CIs were wide due to small sample sizes. A study of 1995–2012 Fatality Analysis Reporting System data reported an aRR of 0.96 (0.90, 1.03) for age 16 and 1.00 (0.93, 1.09) for age 17.[31] With 191,619 injuries, relative to 6,047 fatalities among 16-year-old passengers, the injury analysis had an adequate sample size to detect moderate effects. Restricted driving under graduated driver licensing could increase instances of adolescents riding with parents, and subsequently increase passenger fatalities. However, riding with parents is much safer than riding with adolescents.[22] In addition, graduated driver licensing restrictions likely reduce the number of passengers riding with high-risk adolescent drivers. Some evidence exists that graduated driver licensing may not cause adolescents to ride more frequently as passengers.[32] California adolescents surveyed said that they drove earlier (58%), rescheduled an event (45%), or had parents or other adults as passengers (59%), to adapt to the nighttime driving restriction. Others walked or rode a bus or bicycle (31%). Forty-four percent reported that they drove at night despite the restriction.[32]

In this study, graduated driver licensing did not appear to be associated with more injuries to adolescent bicyclists or pedestrians. In another study of Upstate New York adolescents post- and pregraduated driver licensing, rate ratios of incapacitating injuries were 1.53 (0.89, 2.62) and 1.75 (0.91, 3.34) for 16- and 17-year-old pedestrians and bicyclists, respectively,[10] although the study population might not be representative of 16- to 17-year olds in the 43 states studied. Potential reasons for no increase in bicyclist or pedestrian injuries after graduated driver licensing may include adolescents forgoing or rescheduling trips;[32] walking rather than riding a bus or bicycle, or simply violating the law.[32]

A study of 1992–2002 Fatality Analysis Reporting System data reported that graduated driver licensing reduced total traffic fatalities (drivers, passengers, bicyclists, and pedestrians) by 6% among 15- to 17-year olds.[18] Previous studies have suggested heterogeneity between ages 16 and 17.[14,31] Our study showed that for total injuries (passengers, bicyclists, pedestrians, bus riders, and drivers), graduated driver licensing was associated with a 10% reduction for age 16, but not for age 17. Adult supervision and restrictions on nighttime driving and number of underage passengers are likely common among 16-year olds. As adolescent’s age and restrictions are removed, the influence of graduate driver licensing should decrease. An analysis of 2006–2009 Fatality Analysis Reporting System data found that the nighttime driving restriction was in effect for 84% of 16-year old, and 30% of 17-year-old drivers in fatal crashes.[33] In 36 of 50 states, 17-year olds could have full licensure (unlimited unsupervised driving), particularly after taking a driver education course.[4]

Strengths of this study include examination of nondrivers and drivers, which increases understanding of the broader impacts of graduate driver licensing; longitudinal analysis of injuries from 43 states; examination of injuries, which produces a sample size adequate for analysis of injuries to bus riders; and adjustment for temporal trends, seasonal factors, traffic laws, and economic factors. Limitations include not having licensure status to estimate the effects of individual graduated driver licensing components; and calculations of adjusted economic factors included all ages, not just adolescents. Also, our use of population estimates to calculate rates did not account for exposure to driving, bus riding, bicycle riding, or walking.

ACKNOWLEDGMENTS

We express appreciation to Michele Fields and Laurel Sims at the Insurance Institute for Highway Safety for their assistance in describing graduated driver licensing systems. We thank Herbert Linn at West Virginia University Injury Control Research Center for editorial assistance. These individuals received no compensation for their contributions.

Table.

Counts, Rates, and Rate Ratios for Injuries by Person Type, Age, and Graduated Driver Licensing Exposurea