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A Census of Young Drivers in Iowa


Abstract: Driver licensing and history data are important to consider when analyzing young driver crashes because crash rates decrease steeply as independent driving experience is gained. Since directly measuring or estimating young drivers’ actual exposure to driving in terms of miles, trips, or time duration is problematic, duration of licensure is a common surrogate for driving experience. Another common denominator used to calculate crash rates is the number of licensed drivers. To date, data about the number of young drivers in the State of Iowa holding each type of license and duration of licensure has not been readily available.

Additionally, the effect of different graduated driver licensing (GDL) policies on crash rates cannot be evaluated without knowing which drivers were and were not subject to the policies. Objectives: This project aimed to quantify the number of young drivers in Iowa according to license type (instruction permit, minor school license, intermediate license, full license issued after completing GDL or full license outside of GDL) by gender and age (from 14 to 21 years in quarter-year increments) from 2008 through 2017 (also in quarter-year increments). Then the data tables were analyzed to identify trends in licensing patterns. Methodology: We requested and obtained from the Iowa DOT records for all permits and licenses issued to young drivers in Iowa from 2008 through 2015. A separate data file was generated for each type of driver license and each driver was identified by a unique customer number. After merging the four data files, duplicate records were eliminated, as were records for licenses issued at age 21 or older.

The clean data were tabulated into the datasets described above in the Objectives. Linear regression was applied to quantify licensing trends. Findings: Overall analysis of the licensing trends indicate that proportionally more young drivers are obtaining an intermediate license at the earliest ages. At the same time, more young drivers are bypassing the GDL system and obtaining full licenses after they reach the age of 18. Changes to Iowa’s GDL in 2014 coincided with a temporary disruption in the licensing trends for both minor school licenses and intermediate licenses. While the trends for the intermediate licenses seem to have returned to previous levels, minor school licenses continue to trend upwards. In 2015, nearly 17,000 minor school licenses were issued compared with about 40,700 instruction permits (approximately 40% of drivers). Current work: We are extending this effort to include licensing data from 2016-17. In addition, and most importantly, the data tables output from this effort are being used to assess the impact of various young driver licensing policies on crash rates for subpopulations of Iowa’s young drivers.

Michelle Reyes - Research Associate University of Iowa



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