In addition, early school start times have been associated with drowsy driving in new teen drivers and higher car crash rates. Schools ending early in the afternoon may also increase the risk of engaging in unhealthy, risky behaviors among sleep-deprived adolescents. Sending children to school before sunrise also means they must wait or walk in dark, with low visibility. Sleep deprivation can result in low motivation, difficulty concentrating, restlessness, slowed reaction times, lack of energy, frequent errors, forgetfulness, and impaired decision-making skills. Studies, many spearheaded by Kyla Wahlstrom and her research team at University of Minnesota's Center for Applied Research and Educational Improvement (CAREI), have tied these effects to early school start times, which, in turn, have repeatedly been linked to increased rates of tardiness, truancy, absenteeism, and dropping out. In 2012 a study using data from Wake County, North Carolina, showed that delaying middle school start times by one hour, from roughly 7:30 to 8:30, increases standardized test scores by at least two percentile points in math and one percentile point in reading. The effect was largest for students with below-average test scores, suggesting that later start times would narrow gaps in student achievement. The impact of later start times on school performance—including reduced truancy, absenteeism, and increased overall academic achievement—is approximately double in economically disadvantaged students. This discrepancy may be explained, at least in part, by the fact that privileged students have opportunities to attend private schools (most of which start instruction after 8 a.m.) and/or save time by driving or being driven to school. A 2017 report by the RAND Corporation concluded that delaying school start times to 8:30 a.m. is a cost-effective, population-level strategy that would significantly impact public health and the U.S. economy, with benefits quickly outweighing any immediate costs. After just two years, the study conservatively projected a gain of $8.6 billion to the U.S. economy. After a decade, this gain would increase to $83 billion, and after 15 years to $140 billion, amounting to an average annual gain to the U.S. economy of $9.3 billion. An earlier Hamilton Project Report published by the Brookings Institution concluded that starting class later can be an immediate and inexpensive way to boost health, safety, and achievement for all students. The authors, economists Brian A. Jacob and Jonah E. Rockoff, predicted that starting high schools one hour later, at about 9 a.m., would result in roughly $17,500 in increased future earnings per student in present value—a benefit:cost ratio of at least 9:1 even when changing schedules requires upfront investment to alter bus schedules and/or accommodate later after-school activities.