Economic factors can have substantial effect s on transportation crash trends. This study makes a comprehensive examination of the relationship between the retail gasoline price (including state and federal fuel taxes) and transportation fatal crashes from 2007 to 2016 in the US. Data on motor vehicle , bicycle and pedestrian fatal crashes come from Fatality Analysis Reporting System (FARS) provided by the National Highway Safety Administration (NHTSA) and the gasoline price data is from U.S. Energy Information Administration (EIA). Random effect negative binomial re gression models are used to estimate the impact of inflation -adjusted gasoline prices on trends of transportation fatal crashes. Initial results combined with results of previous studies showed that gend er and transportation mean type (motorcycle, non-moto rcycle , bicycle and pedestrian ) play prominent roles in interpreting the final model, so by using random effect negative binomial regression, seven models are developed to evaluate the effects of gasoline price changes on total populatio n, male, female, mo torcyclists, non-motorcyclists , bicyclists and pedestrians separately. Our findings suggest that increasing the gasoline prices will not significantly alter the number of total fatal crashes. However, by looking at different vehicle types, it is estimated that one dollar increase in adjusted gasoline price is associated with 24.2% increase in the num ber of motorcycle fatal crashes, 1.9% decrease in the number of non -motorcycle fatal crashes , and 0.7% decrease in the number of pedestrian fatal crashes . Also, there is no noticeable difference between male and female in response to the gasoline price changes. Keywords : Gasoline prices, Traffic safety, Traffic crashes, Fatal crashes, Motorcycle crashes Safaei, Zhou 3 INTRODUCTION Transportation safety relies on numerous parameters ranging from pavement and infrastructure condition to the efficiency of the regulations set by the responsible authorities and their psychological implications on the road users . Recent changes in gasolin e prices have led to convoluted social implications for commuters and decision makers (1). Higher gasoline prices have much more effects than just costing more to fill up the gas tank; it has a diverse range of effects on the broader economy and travel tre nds. With increased gasoline price people tend to drive less to conduct their purchases. People feel good about the economy when the gas price is lower and they tend to spend more on personal consumption such as shopping and dining (2). When gas price surg es, people stop spending and businesses suffer from it. It has been reported that as the gasoline price increased in April 2011 , online shopping in the United States reached its fastest rate in four years (2). Also, Marin Software reported that searches f or online shopping increase significantly along with gas price (2). On the other hand, as the gasoline price rises, retailers need to increase the shipping cost as their vehicles need to spend more on gas. The increase in gasoline price effects the automob ile industry by making automobile manufactures create small, more fuel -efficient vehicles like hybrids (2). Gasoline price when headed to 3.5 dollars in 2011 , led to dramatic psychological effects on consumers (3). Rapid increase in oil price could derail the economic recovery and put into a recession (3). This argument is based on the fact that 10 out of the last 11 post -World War II recessions happened immediately after oil price spikes (3). David Rosenberg stated that one penny increase in the gasoline price sucks 1.5 billion dollars out of annual household cash flow and drags down the GDP growth (3). In July 2008, gasoline price reached a record of 4 dollars per gallon and instantly people made an intense shift to buy smaller, more fuel -efficient cars (4). Hybrid car sales increased by 34 percent during the first season of 2011, in comparison to the first season in 2010 (4). Since the last season of 2014, as gasoline prices began falling, Americans moved from buying mor e fuel -efficient cars to larger pick-up trucks and SUVs (4). According to HIS, SUV and crossovers accounted for 40 percent of US market share in 2016 versus 34 percent in 2014 (4). It is also been noted that gasoline price ranges can have an effect on some public transportation ridership. Based on a report by the American Public Transportation Association, in April 2011, the Raleigh -Durham -Chapel Hill region of North Carolina represented an 18 percent increase in express bus riders compared to the same peri od in 2010 (2). Also, riders on New Mexico’s Rail Runner increased by 14 percent during the same time (2). Gasoline price changes may affect the working schedule of industries. Some businesses with more flexible working schedule have adopted four-day weeks to limit their employees’ financial burden for commuting during the gasoline price rising time (2). Also, some economists warn that gasoline price increase negatively affect the economic recovery; they have mentioned that businesses have re -evaluated their hiring strategies tending to have more l ay-offs due to the uncertain economic condition. Increase in gasoline price can diminish the number of freelancers as it can limit the geographical region in which the business can flourish (2). Gasol ine price can also affect the land and property values. According to a study conducted by Neill at the University of Nevada, based on 930,702 home sales data in the Las Vegas area for a span of more than 30 years, a 2 percent price bump is observed for hom es close to the city center and a 1 percent price drop was observed for homes distant from the city center due to a 10 percent increase in gasoline prices (2). Gasoline price changes may also make people relocate to the areas closer to their workplaces (5, 6). It might be possible that as the gasoline price increases, travelers change their transportation modes and reduce their driving frequency, leading to a reduction in gasoline consumption (1); people might shift to using motorcycles, bicycles or public transit instead of their Safaei, Zhou 4 four-wheel vehicles ; they might even choose to walk instead of riding or driving ( 7). In 2008, Mattson created a dynamic model for studying the short and long -term effects of gasoline price changes on transit -ridership use. He foun d that in large and large -medium urban areas, the increase in public transit -ridership in response to the gasoline price rise is considerable and it happens immediately after the change , however for smaller areas due to the less familiarity of people with public transportation systems , it took five to seven months for them to respond to the gasoline price changes and to use public transit systems instead of their personal motor vehicles (8). Gasoline price changes can also play an important role in transportation safety. Chi et al.’s findings suggested that if decision makers wished to reduce traffic crashes, increasing gasoline taxes would represent a possible option due to the relations hip between higher gasoline prices and traffic crashes (9, 10). A comprehensive literature review was conducted on examining the effects of gasoline price changes on traffic safety. Chi et al. had studied the effects of gasoline prices on crash numbers. Us ing both the Poisson -gamma regression model and Prais -Winsten linear regression model, they found that when the inflation -adjusted gasoline price increased 1%, the monthly total crash number decreased 0.25% in the short term and 0.47% at a one -year lag. Ho wever, the gasoline price changes had no effects in the long term; they also found that higher gasoline prices reduced crashes for younger drivers immediately and at a one -year lag for older and male drivers. They also claimed that the gasoline price incre ase had both immediate and one -year-lag effects on females, whites, and blacks (9). In another study conducted by Chi et al. in 2013, they examined the relationship between the fluctuations in gasoline prices and traffic safety using monthly traffic crashe s in the state of Mississippi from April 2004 to December 2010. They found that gasoline prices had a stronger negative correlation with less severe crashes. The effects of the gasoline price changes took place at a nine -month lag, peak at a twelve -month l ag, and weaken after an eighteen - month lag. Furthermore, the results indicated that the gasoline price fluctuations had negligible effects on reducing fatal crashes. They also concluded that gasoline prices did not have an immediate effect on crash rates o f male drivers. They hypothesized that as men’s trips were more work -related (11), it would take a longer time for them to react to the new policy of gasoline prices. Women in Mississippi were responsible for more household activities, and they made more s hort- distance trips so they could easily change their frequent short -distance trips to less frequent multi - purpose trips (1). In another study conducted by Chi et al., they focused on the differences between the relationship of gasoline price and traffic c rashes in urban and rural areas using the monthly traffic crash data from 1998 to 2007 at the county level in Minnesota. They found that although fatal crashes in both the urban and rural areas were strongly affected by the gasoline prices and similar decr eases were observed when gasoline price increased, total crashes, property -damage - only crashes, and injury crashes were greater affected by the gasoline price changes in rural areas than urban areas (12). Also, in 2011, Chi et al., using the Mississippi mo nthly crash data from 2004 to 2008, studied the connections between gasoline prices and drunk -driving crashes. In this study, they found that higher gasoline prices would lead to fewer drunk -driving crashes. They also realized that less severe crashes were more strongly affected by gasoline price changes than fatal crashes while higher alcohol consumption may lead to more severe crashes (13). Chi et al., mentioning the limitations of other studies focusing either on fatal crashes only or all crashes being measured over a short period, conducted another study to examine gasoline price effects on all traffic crashes by demographic groups in the state of Alabama from 1999 to 2009 (10). They estimated the effects of gasoline price changes on automobile crash tre nds. They used age group, gender and race/ethnicity as dependent variables. They found that gasoline price changes had a Safaei, Zhou 5 stronger effect in reducing crashes involving young drivers (aged 16 -20) than the older drivers in the short term. They asserted that t he reasons of such an observation might be the tight budget of teenagers that made them more vulnerable to gasoline price changes, or either the rigidity of elders due to their fixed work driving routes and family responsibilities. They declared their find ings to be consistent with those from some prior studies (9, 14). Regarding gender, they found a contradiction between their results and their findings in the previous study (9). In the Alabama study (10), they found that there was no significant differenc e between male drivers and female drivers in both the short -term and long -term perspective, however, in the Mississippi study (9), they found that increasing gasoline prices more strongly reduced crashes involving female drivers than male drivers. Also, re garding race/ethnicity, the two studies did not come up with the same results; in the Mississippi study (9), they found that crashes involving non -Hispanic white drivers were more vulnerable to gasoline price changes than crashes involving non -Hispanic black drivers in the short term and the reverse in the long term. However, in the Alabama study (10), neither in the long term nor in the short term, there were differences between the effects of gasoline changes on crashes involving non -Hispanic white drivers and non -Hispanic black drivers. In a recent study, Chi et al., by analyzing Mississippi traffic crash dat a from 2004 to 2012, found that roadway safety would react to the gasoline prices in a nine to ten -month lag regardless of age, gender and races with some exceptions (15). Regarding the effects of gasoline taxes, a paper was written by Grabowski et al. to understand the impact of gasoline taxes on the traffic fatality rate. They used a semi -log model on the total number of traffic fatalities for the 48 states from 1983 to 2000 with the total of 912 observations. They found that the traffic fatality rate dro pped by 0.6% with a 10% increase in the gasoline tax; they also argued that the tax that led to a noticeable increase in the gasoline price would decrease traffic fatalities considerably (14). In another paper by Leigh et al., they found that people drove less or slower with a higher gasoline tax, which in turn reduced fatalities. They mentioned that gasoline taxes affected fatalities indirectly through decreasing speeds and fewer miles driven. The study found that, as gasoline taxes increased by 10%, the f atalities were expected to drop by 1.8 to 2.0%. However, the long -term effects may not be as evident as the results found. In addition to reducing fatalities, tax revenues could be invested in mass transit systems which in turn would increase traffic safet y. The bipartisan National Surface Transportation Infrastructure Financing echoed this idea in raising the federal gasoline taxes from 18.4 cents to 28.4 cents per gallon to increase highway funding and reduce budget deficits (16). There is also a consider able amount of literature that had studied the effects of gasoline price changes on the number of fatalities instead of crash numbers. Using a panel data model, Ahangari et al., incorporated other economic factors and found that with a 10% increase in the gasoline price, traffic fatalities decreased by 2.18%. According to the study, if the unemployment rate increased by 10%, the traffic fatalities would be expected to reduce by 0.65%. Additionally, the road fatality rate decreased by 41.5% with a 10% increa se in the health index factor (17). Zhu et al. claimed that an increase in gasoline price is accompanied by the increased costs of hospitalization of motorcycle injuries. In one study, by using the data on inpatient hospitalizations, obtained from the Nati onwide Inpatient Sample between 2001 and 2010, they built panel feasible generalized least square models to examine the association of gasoline prices with hospital utilization and cost for motorcycle and non -motorcycle motor vehicle crash injuries. They f ound that unless the increase in the gasoline tax was integrated with public health intervention policies to improve the motorcycle safety, increased gasoline prices would cause as much rise in hospitalization cost of motorcyclists that would compensate fo r any reductions in the Safaei, Zhou 6 number of non -motorcycle injuries (18). In a prior study, Zhu et al., had chosen California as their study field as California had the highest number of motorcycle registrations in the USA, and it was ranked third in number of motor cycle crashes which accounted for the 8% of all motor vehicle fatalities and 13% of injuries from motorcycle crashes in the USA. By using the Statewide Integrated Traffic Records System for 2002 –2011, autoregressive integrated moving average (ARIMA) regres sion models were utilized to estimate the impact of inflation -adjusted gasoline price per gallon on trends of motorcycle injuries. Their findings suggested that raising gasoline prices increased the number of motorcycle riders on the roads and, consequentl y, caused more motorcycle injuries. They indicated that besides the mandatory helmet use law, implementing ways to raise risk awareness of motorcyclists, investment in alternative transportation modes like public transportation and making strict licensing tests of riding skills would help to reduce the motorcycle fatal and non -fatal injuries (19). Hyatt et al., conducted a study to evaluate the association between increases in gasoline price for automobile occupants and motorcycle riders and motor vehicle collision -related injury and fatality rates; they reported that although the number of injuries and fatalities in motorcycle -related crashes increased with increasing gasoline price, rates of motorcycle -related injuries and fatalities per registered vehicle s remained unchanged. They concluded that it was less probable that raising the gasoline prices would affect the driver's characteristic that might result in more crashes. Instead, they told that it made more drivers change their transportation modes to mo torcycling and the increased number of motorcyclists on the road was the primary cause of increased motorcycle -related fatalities and injuries (20). Wilson et al., mentioning that raising the fuel price was a probable reason for the popularity of motorcycl ing in the USA, quantified the relationship between changing the fuel price and motorcycle fatalities. Their findings suggested that in response to the gasoline price increase and to reduce their fuel expenses, people aimed to change their modes of transpo rtation to motorcycles, and although the fatality rates of automobile crashes declined as a response to the gasoline price increase, motorcycle fatality rates had risen. They mentioned the fact that they did not study changing p