CHAPTER 1

Background

Teenage drivers are overrepresented in motor vehicle crashes. According to the latest data available from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System, 2,883 teenagers aged 13–19 were killed in motor vehicle crashes in 2022 (Insurance Institute for Highway Safety 2024). The crash risk was highest for drivers aged 16–17, compared to other age groups. In passenger vehicles, 56% of teenage passenger fatalities occurred when another teenager was driving; 51% of crash deaths occurred on Fridays, Saturdays, and Sundays, with the highest occurrence between 9 p.m. and midnight.

Crash rates during the learner’s permit period are low and are similar to crash rates for adult drivers (Gershon et al. 2018a). When teenagers start learning to drive independently, crash risk dramatically increases at licensure and remains elevated during the first several months of independent driving (Simons-Morton 2007). A range of reasons for high crash rates during teen drivers’ early independent driving have been proposed in previous studies (McCartt et al. 2009; Romer et al. 2014; Simons-Morton 2007; Simons-Morton et al. 2011), including inexperience, distraction, night driving, risky behavior, and teen passenger presence. Age, driving experience, and exposure have been consistently considered as major contributing factors to crash risk (McCartt et al. 2009; Simons-Morton et al. 2017).

Few studies have provided empirical evidence on the effect of adult-supervised practice driving, measured by the number of practice hours, on crash risk among newly licensed drivers (Winston et al. 2015). To bridge the gap, Ehsani et al. (2020) investigated the driving exposure or amount of practice during the learner’s permit period and safety outcomes during independent driving. Results showed that consistent driving practice during the learner’s permit period (rather than intensive practice just prior to the examination) could reduce teen drivers’ crash risk during the first year of independent driving.

Driving exposure can be defined based on driving miles or hours but it can also include exposure to various road conditions and weather, as well as passenger presence and safety-critical events (SCEs). Generally, as exposure (i.e., cumulative driving hours or miles) to a variety of road/traffic conditions increases, the crash risk tends to rise (Elvik 2010; Simons-Morton 2007). However, teen drivers require practice to improve their driving skills. Klauer et al. (2011) suggested that crash rates may be higher when teen drivers are exposed to known risk factors (e.g., nighttime, teenage passengers, speeding). The relationship between amount of practice and crash rates is largely unknown.

It is not generally known whether learners obtain the amount of required practice driving during the learner’s permit period; nor is it known whether they gain exposure to a variety of driving situations. Research has suggested that a substantial number of practice hours should occur in diverse environments (e.g., commercial districts, country roads, or highways) and conditions (e.g., nighttime, weather) to enhance driving skills and reduce crash rates (Winston et al. 2015).

A small but compelling body of literature suggests that teenage drivers who obtain driving experience in a broad range of driving environments have fewer crashes than those who do not receive such training (Mirman et al. 2014; Simons-Morton et al. 2017; Ehsani et al. 2020). A web-based intervention aiming at increasing teenagers’ diversity of practice driving and improving driving performance before licensure has been developed (Mirman et al. 2014; Winston et al. 2015).

However, further research is necessary to fully understand how driving exposure and diversity during the learner’s permit stage and/or early independent stage affects subsequent crash risk. This project analyzed two naturalistic driving studies (NDSs)—the second Strategic Highway Research Program (SHRP 2) NDS and the Supervised Practice Driving Study (SPDS)—to evaluate how exposure to driving in more diverse traffic and road environments is associated with teen driver behavior.

This project addressed the following research objectives:

1. Evaluate how exposure to driving in diverse traffic and road environments earlier in the learning-to-drive process is
   1. Associated with crashes and/or other safety surrogates (e.g., near-crashes, elevated g-force events) in later independent driving.
   2. Associated with variability in teen driver behavior or safety-relevant performance measures in later independent driving, corrected/normalized for exposure.
2. Develop recommendations/strategies for improving teen driving safety for State Highway Safety Offices.