Data source and study population
Data was obtained from state Youth Risk Behavior Surveys (YRBSs), which are repeated cross-sectional surveys using a two-stage cluster sample design. State YRBSs are anonymous, voluntary surveys conducted every two years to obtain a representative state sample of 9th through 12th grade students attending public school. 7-9
The YRBSs sampling design and methodology for combining and analyzing state-level data has been described previously.7-9,31 Only state with a response rate ≥ 60% would be weighted and access to public. States that asked a question about talking on a cellphone while driving in at least one of the years from 2013 to 2017 were included in this analysis. Participating states are listed in Additional File Table 1. The study inclusion criteria were students who had reached their state’s minimum age to obtain a learner’s permit license and had driven at least once 30 days prior to the survey administration date.
State cellphone bans and components of their graduated driver license system (GDL) were obtained from the Insurance Institute for Highway Safety.20,32 The ban amendments and effective dates were identified using the LexisNexis Academic database and state legislative documents.33 The total number of public school districts and the number of districts in rural areas were obtained from the National Center for Education Statistics.34 Detailed values of these variables for each state are listed in Additional File Table 2.
The study outcome was self-reported talking on a phone while driving (we used calling while driving, CWD as an abbreviation not TWD because TWD usually refers to texting while driving), which was measured with the question: “During the past 30 days, on how many days did you talk on a cell phone while driving a car or other vehicle?” Response options included seven ordinal categories ranging from 0 to 30 days. Students who responded “I did not drive” were excluded from the analysis. For the descriptive analysis, we categorized responses into never (0 days), sometimes (1-9 days) and frequent (10-30 days) engagement in CWD. For multivariable analysis, we created a binary outcome (never versus at least once) as any exposure to talking on a phone while driving may increase crash risk for teen drivers. A similar categorization was consistent with a previously published study using YRBSs data on texting/emailing while driving.35
The state status of handheld calling bans and young driver bans were classified as 1) the absence of a handheld calling ban and a young driver ban (no ban); 2) the absence of a handheld calling ban but an enacted young driver ban (young driver ban); and 3) the enactment of both a handheld calling ban and a young driver ban (concurrent bans). No YBRSs participating state had an enacted handheld calling ban but an absence of a young driver ban during the study period. Cellphone law information for each state is listed in Additional File Table 2.
Previous studies have reported that teen driver cellphone use, varies by age, sex, race/ethnicity, and urban/rural status.23,27,35-37 For our study, urban/rural status was presented by the proportion of state’s public school districts that were in rural areas, which was calculated by dividing the number of public school districts in rural areas by the total number of public school districts in that states. We categorized the district proportion into three groups by approximately equal tertiles (17%-50%; 51%-78%; 79%-90%). Given that studies have shown states that implemented stronger GDL systems had larger reductions in the frequencies of teen driver crashes and rate related injury and fatality, state GDL system strength was included.38-42 The strength of the GDL system for each state was based on scores suggested by Steadman et al., 43 and categorized as: fair to moderate (<8 points); good (=8 points); and excellent (>8 points; see Additional File Table 2).
The association of cellphone laws and CWD was examined by adjusting for student demographics, the state GDL score, the proportion of state’s public school districts in rural areas, and survey year. Crude and adjusted prevalence ratios (PRs) with 95% confidence intervals (CIs) for CWD were estimated using Poisson regression models with robust variances estimation.44 Complete case analysis was used to handle missing data as the percentage of missing was small (approximately 3% of students reached permit age but did not answer the question on CWD).
All reported results were weighted and considered the survey design.31 Data analyses were performed in 2019 using SAS Enterprise Guide 7.1 (SAS Institute Inc., Cary, NC) and STATA 14.0 (StataCorp LLC, College Station, TX).
Several sensitivity analyses were conducted to assess potential bias: 1) restricting the analysis to the five states that participated in all three survey years (Connecticut, Massachusetts, Missouri, Montana, North Dakota); 2) excluding Utah, which enacted their young driver ban during the same year the survey was conducted (2013), thus limiting the sample to states that enacted young driver bans before survey administration; 3) excluding Texas, which was weighted as 41% of the total study population (the total population in Texas is much larger than other participating states); 4) excluding Maryland, which conducted a census instead of a two-stage cluster survey.
We further restricted our analysis to students who had reached the state-dependent age to begin unsupervised driving under certain driving conditions as driving under the supervision of an adult driver may prohibit teen’s CWD behavior.29,32 Lastly, to estimate how cellphone laws are associated with a nominal outcome, we fitted Poisson regression models to estimate the prevalence ratio for 1) sometimes CWD vs. never CWD, and 2) frequent CWD vs. never CWD.