Average mileage per month for teenage drivers was 366 miles, an average of over 6000 miles for the 18 month study period. Parents drove a mean of 3000 miles with a median of 1058 miles during the study period.
Teen-Parent Comparison of Crashes and Near Crashes
There were 37 crashes and 242 near crashes among teenage participants for the entire 18-month study period; parents were involved in 2 crashes and 32 near crashes.32
Crashes among teenagers included four that resulted in police reports and one that resulted in injury for which hospitalization was not required.
Video footages of a sample of elevated g-force events were reviewed to assess the percentages of valid events vs. those that can be attributed to potholes or other road conditions. Accordingly, 788 of the 816 (96.6%) hard stops, 1,012 of the 1,065 (95.0%) rapid starts, 576 of 576 (100.0%) hard left turns, and 704 of 709 (99.3%) hard right turns were determined to be valid. Of the valid events, 79.1% of hard stops, 95.8% of hard starts, 91.0% of hard left turn, and 79.1% of hard right turns were attributed to misjudgment. Based on these findings, all events were included in analyses.
Spearman rank order correlations between individual rates of risky driving and CNC rates for teenage participants were 0.75 for rapid starts, 0.76 for hard stops, 0.53 for hard left turns, 0.62 for hard right turns, 0.46 for yaw, resulting in a correlation between CNC and the composite measure of risky driving of 0.60.
CNC and Risky Driving Rates by Passengers, Night, Driver Sex and Psychosocial Factors
Shown in are the properties of the psychosocial variables. With the exception of two sensation seeking subscales, all alpha values were ≥ 0.80.
Psychosocial measures: number of items, range, means, standard deviations, and scale alpha coefficients for teenage participants.
Separately for CNC and the composite measure of risky driving, univariate models were fit to assess the relationships with passenger presence, night driving, driver sex, sensation seeking, substance use, and risk-taking friends. For each dependent variable, each covariate was examined separately in a random effect Poisson regression model adjusted for time since licensure (in quarters). Further, the interaction between each covariate and time since licensure was also examined. The univariate findings are shown in . P values based on likelihood ratio tests indicate the significance of the IRR’s for the overall study period (p1) and whether the quarterly IRR’s are identical (p2).
Univariate analyses of covariate associations with CNC and the composite measure for teenage risky driving with estimates and tests of IRRs overall and quarterly.
The CNC rate among teenage drivers was lower in the presence of adult passenger (IRR=0.25) and higher among teenage drivers with more risk-taking friends (IRR=1.96), as shown in and . CNC rates did not vary significantly by night, driver sex, or driver substance use.
Quarterly median incidence rates (IR) for crashes and near crashes (CNC) (a) and the composite risky driving measure (b) among teenage drivers with no passengers (solid line), teenage passengers (dashed line), and adult passengers (dotted line).
For risky driving, the presence of adult and teenage passengers (IRR = 0.33 and 0.82, respectively) and driving at night and late at night (IRR = 0.80 and 0.94, respectively) were negatively associated with risky driving. Risky driving was significantly lower among both male and female teenage drivers in the presence of both male and female teenage passengers (data not shown). In contrast, having more risk-taking friends (IRR = 2.09) was associated with higher rates of risky driving, while driver substance use was not. The IRRs varied over time for all measures except adult passengers. Risky driving by passenger condition is illustrated in .
In multivariate analyses, shown in , CNC was significantly associated with adult passengers (IRR=0.26) and risk-taking friends (IRR=1.87), while risky driving was negatively associated with adult (IRR= 0.32) and teen (IRR= 0.81) passengers and early night (IRR=0.81) and positively associated with risk-taking friends (IRR=1.97). Under the assumption that passenger effects would be greater among teenage drivers with risky friends, we examined the interaction of risky friends on the relationships between teenage passengers and risky driving, but the interaction was not significant (data not shown).
Multivariate analyses of covariate associations with CNC and the composite measure for teenage risky driving for the overall study period, with simultaneous adjustment for quarter, time of day, passengers, and risky friends.