Transportation in the context of this HIA focuses on issues related to the school commute of students. The school commute encompasses the trips and travel modes—car, bus, walking, bicycling—used to transport students to and from a school location. For this HIA, the school commute focuses on those traveling to and from public schools.

Many factors such as distance from home to school, school policies (e.g. parking fees and bus fees), land use context (urban vs. suburban vs. rural), and traffic safety influence the transport mode a student uses to get to and from school, such as walking, biking, school bus, or car (Giles-Corti et al. 2009). Distance has been shown to be a strong predictor for active transport. For example, it has been found that students who live a half-mile or less from their school are more likely to walk or bike than those who live farther away (Timperio et al. 2006).

The mode of transportation that students use has implications for their health. It affects their potential exposure for vehicular crashes and exposure to environmental factors such as air pollution as well as health risk behaviors.

School Commute and Physical Activity:
In 1969, nearly half of students aged 5-18 in the US walked or biked to school. By 2001, only slightly more than one in ten commuted by foot or bicycle (M. C. Lee, Orenstein, and Richardson 2008a; Bungum et al. 2009; Eyler et al. 2008), with most of these trips being replaced by vehicular trips (e.g. drop-off, school buses). Similarly, the percentage of US Kindergarten to eighth grade school students who live less than a mile from their school fallen almost by half (45% to 23%) (M. C. Lee, Orenstein, and Richardson 2008b). During this same period, the percentage of students who are overweight and obese has increased. In 2009, 14% of high school students in Massachusetts were overweight and 11% were obese (CDC Obese Youth Over Time, 2014) – percentages that have remained relatively constant for almost 10 years. Overweight and obesity predispose youth to future overweight and obesity, as well as a variety of risk factors, conditions, and chronic diseases like Type II diabetes and cardiovascular disease (CDC, 2014).
A number of studies have examined the connection between commute modes and physical activity, with most finding a positive association between active commuting to school and level of physical activity among students (Davison, Werder, and Lawson 2008). This research has shown that students who walk or bicycle to school have higher daily levels of physical activity and are more likely to meet physical activity rec¬ommendations than those traveling to school by car or bus. Physical activity is protective against becoming overweight and obese (M. C. Lee, Orenstein, and Richardson 2008a); however, we were not able to identify any studies that looked specifically at the impact walking to school has on the rates amongst students through our literature review. Physical activity also protects against other physical and psychological risk factors and illnesses. Regular physical activity has been found to reduce risk for some cancers and developing type 2 diabetes, and it can improve an individual’s mental health and mood, decreasing risk of depression. Among students, those that are more physically active – in and outside of school – are more likely to perform better academically (Trost 2007).
Research investigating elements of the physical environment suggests that students are more likely to walk or bicycle when the route to school is direct, there are few steep roads on the route, and neighborhoods surrounding the student’s home are considered walkable (e.g., by measurements of residential density, intersection density, and land use mix) (Timperio et al. 2006; Kerr et al. 2006; McMillan 2007). In the social context, the literature shows that public school students are more likely to actively commute to school than private school students and suggests that students from low socioeconomic (SES) backgrounds are more likely than students from high socioeconomic SES backgrounds to walk or bike for their school commute. Research also shows that students are more likely to have an active school commute when their parents perceive that other students in the area walk or bike to school (Timperio et al. 2006; Y. Yang et al. 2010), and other family members agree with allowing a student to walk or bicycle to school (McMillan 2007).

School Commute and Traffic Safety:
Motor vehicle crashes are the top cause of death among people ages 5 to 34 in the United States, and a leading cause of injury among all age groups (Centers for Disease Control and Prevention 2011). Each year approximately 800 school-aged children are killed in motor vehicle crashes during normal school travel hours. Normal school travel hours are defined as 6am to 8:59am and 2pm to 4:59pm. Of these 800 deaths, about 20 (2%)—5 school bus passengers and 15 pedestrians—are school bus-related. The other 98% of school-aged deaths occur in passenger vehicles or among pedestrians, bicyclists, or motorcyclists. A disproportionate share of these passenger vehicle–related deaths (approximately 450 of the 800 deaths, or 55%) occurs when a teenager is driving (National Academy of Sciences 2002). Additionally, approximately 152,000 school-age children are non-fatally injured during normal school travel hours each year. More than 80% (about 130,000) of these nonfatal injuries occur in passenger vehicles; only 4% (about 6,000) are school bus-related (about 5,500 school bus passengers and 500 school bus pedestrians), 11% (about 16,500) occur to pedestrians and bicyclists, and fewer than 1% (500) are to passengers in other buses (National Academy of Sciences 2002).
Vehicular crashes are the leading cause of death for teens in the United States(CDC 2014). The most at-risk group for crashes is teens ages 16-19, who are 3 times more likely to crash than those over the age of 20 (CDC 2014). Teen drivers tend to underestimate hazardous conditions, are more likely to speed, and have the lowest reported rates of seatbelt use. Only 54% of US high school students reported wearing a seatbelt (CDC 2014).
Pedestrians and bicyclists are particularly vulnerable for injury or fatality from motor vehicle crashes. For example, if a pedestrian is struck by a vehicle traveling 20 mph, the risk of a fatality is 6%; at 30 mph, the risk rises to nearly 20%, and at 45 mph, the risk of the pedestrian being killed is 65% (Health Resources in Action 2013). Based on 2012 crash data, 6% of pedestrian crash fatalities and 18% of pedestrians injured in traffic crashes were children under the age of 15 (NHTSA 2014a). Among bicyclists, children age 15 and under accounted for nearly 10% of all fatalities involving bicycle crashes and 20% of the bicyclists injured in crashes in 2012(NHTSA 2014b).
School bus transportation offers increased safety over driving. Buses tend to have lower crash rates as well as less injuries, when compared to other vehicles (J. Yang et al. 2009). Some reasons cited for the increased safety of school buses are the flashing red lights and stop sign that alert drivers to students crossing the street. Additionally, buses are brightly colored, have crush standards as well as reinforced sides, are equipped with multiple mirrors, and drivers are screened and trained more extensively than other drivers on the road (NHTSA).

Traffic Congestion:
Recent research has focused on differences in traffic patterns and specific car characteristics, and how these influence air quality. Traffic congestion can occur during rush hours (morning and evening) and in work zones, and it is under these conditions that carbon dioxide (CO2) emissions and gasoline usage peak (Zhang et al., 2011a). Research has also demonstrated that car and bus idling frequently occurs outside of schools, especially in the morning, and this can significantly contribute to traffic congestion and emissions. One 2011 study that looked at National Household Travel Survey data (McDonald et al. 2011) reported that 7% of morning rush hour vehicle miles traveled and 10-14% of all vehicles on the road are attributable to school transportation. Car emissions that contribute to poorer air quality near schools also contribute to asthma cases in children (McConnell et al. 2010).

Traveling in Personal Vehicles:
Studies of air pollutants have found that in-vehicle concentrations are higher than ambient concentrations for most pollutants measured (HEI 2010; Brugge, Durant, and Rioux 2007b; Zuurbier et al. 2010). For example, in one study it was found that in-vehicle average concentrations of PM2.5 and CO were 2.5 and 6 times higher than concentrations measured at nearby sites (HEI 2010). A study of a low density city found that CO concentrations were highest in cars, compared to public transit buses and bicycles, while ultra-fine particle concentrations were highest among public transit bus and car occupants (Kingham et al. 2013). This may be because the high pollutant concentration is due to the proximity of the in-car microenvironment to the sources of air pollution - vehicles on the roadway.
The air exchange rate in cars is much higher than the rate in buildings, meaning that air pollution flows into cars easily, with the car offering little protection. This phenomenon holds regardless of the type of vehicle and ventilation settings (HEI 2010). The main factors impacting air pollution in a vehicle are the quantity of exhaust emitted immediately in front of a vehicle, and roadway congestion. Closely following a heavily emitting vehicle can increase air pollutant concentrations in the car behind. For example, following a diesel-powered truck or bus could double short-term PM concentrations inside the vehicle immediately behind (HEI 2010). Recent studies have suggested that using air conditioning with air filters can improve in-car air quality and modify the negative health effects of particulate matter (Chuang et al. 2013).

Traveling in School Buses:
Studies have shown that some of a conventional school bus’s own exhaust ends up in the bus cabin, or in other words, that buses can be self-polluting (Sabin et al. 2005), though there is some ambiguity about the primary source of contaminants in buses (Borak and Sirianni 2007). The literature indicates that passengers in school buses may be more exposed to air pollutants and that the air pollution during the bus rides can be a significant portion of total daily pollution exposure for youth (Behrentz et al. 2005; Adar et al. 2008, Ireson et al. 2011). This exposure may also vary in relation to density of surrounding land uses as another study found that the pollutant levels were higher on urban routes than rural/suburban routes (Sabin et al., 2005).
Many studies have shown currently available control technologies can nearly eliminate particulate self-pollution (Adar et al. 2008; Borak and Sirianni 2007; Zhang and Zhu 2011). In terms of the cost-benefit of retrofitting, Marshall & Behrentz (2005) found that even if emission reductions were more expensive per gram emitted for school buses than for a typical vehicle, it would still be cost-effective, due to the high volume inhaled by students in bus cabins. In addition to benefitting the children riding school buses, the community health impacts of retrofitting are potentially significant. A study in Washington analyzed retrofits involving DOCs and crankcase ventilation [“CCV”] filter retrofits, and found that districts adopting the DOC retrofits experienced 5.4 fewer asthma and bronchitis cases per 100,000 children per month and 1.8 fewer asthma and bronchitis cases for those with chronic conditions per 100,000 adults per month (Beatty and Shimshack 2011).

Active Transportation and Near Roadway Exposures:
Those walking and biking are also exposed to vehicular emissions. Although identifying the source of the emissions can be difficult, those who walk or bicycle along higher volume roadways can be directly affected by TRAP. In particular, cyclists can potentially experience greater exposure to emissions due to their higher rates of respiration as compared to those traveling by other modes (e.g., pedestrians, drivers) (Jarjour et al. 2013; Zuurbier et al. 2010; Chertok et al. 2004).
Exposure can be reduced for pedestrians and bicyclists through the use of lower traffic volume routes or paths that are located away from higher volume roadways (Jarjour et al. 2013) . Additionally, some studies suggest that even if walkers or bikers use routes with higher vehicular traffic volumes, the benefits of increased physical activity can outweigh risk posed by pollutants (Grabow et al. 2012, Hartog et al. 2011). For example, Hartog and colleagues (2010) found that for individuals who shift from car to bicycle the benefits of increased physical activity are substantially larger (3-14 months gained) than the potential mortality effect of increased inhaled air pollution does (0.8-40 days lost) and the increase in traffic crashes (5-9 days lost) in terms of life-years gained or lost.
Other studies have noted that increasing the number of active commuters in general has the potential to reduce exposure for those who walk or bike. Modeling the shift of commuters from vehicles to foot or bicycle or shifting to low emission and electric vehicles is estimated to produce a reduction of vehicular emissions and traffic along local routes, thus reducing potential for exposure (David Rojas-Rueda et al. 2011, D Rojas-Rueda et al. 2012, Grabow et al. 2012).