Road incidents are caused by a large number of human factors such as failing to act according to weather conditions, road design, signage, speed limits, lighting conditions, pavement markings, and roadway obstacles. A 1985 study by K. Rumar, using British and American crash reports as data, suggested 57% of crashes were due solely to driver factors, 27% to the combined roadway and driver factors, 6% to the combined vehicle and driver factors, 3% solely to roadway factors, 3% to combined roadway, driver, and vehicle factors, 2% solely to vehicle factors, and 1% to combined roadway and vehicle factors. Reducing the severity of injury in crashes is more important than reducing incidence and ranking incidence by broad categories of causes is misleading regarding severe injury reduction. Vehicle and road modifications are generally more effective than behavioral change efforts with the exception of certain laws such as required use of seat belts, motorcycle helmets, and graduated licensing of teenagers.
Human factors Human factors in vehicle collisions include anything related to drivers and other road users that may contribute to a collision. Examples include driver behavior, visual and auditory acuity, decision-making ability, and reaction speed. A 1985 report based on British and American crash data found driver error,
intoxication, and other human factors contribute wholly or partly to about 93% of crashes.
Drivers distracted by mobile devices had nearly four times greater risk of crashing their cars than those who were not. Research from the Virginia Tech Transportation Institute has found that drivers who are texting while driving are 23 times more likely to be involved in a crash as non-texting drivers. Dialing a phone is the most dangerous distraction, increasing a drivers' chance of crashing by 12 times, followed by reading or writing, which increased the risk by ten times. An
RAC survey of British drivers found 78% of drivers thought they were highly skilled at driving, and most thought they were better than other drivers, a result suggesting overconfidence in their abilities. Nearly all drivers who had been in a crash did not believe themselves to be at fault. One survey of drivers reported that they thought the key elements of good driving were: Accompanying changes to road designs have been wide-scale adoptions of
rules of the road alongside law enforcement policies that included drink-driving laws, setting of speed limits, and speed enforcement systems such as
speed cameras. Some countries'
driving tests have been expanded to test a new driver's behavior during emergencies, and their hazard perception. There are demographic differences in crash rates. For example, although young people tend to have good reaction times, disproportionately more young male drivers feature in collisions, with researchers observing that many exhibit behaviors and attitudes to risk that can place them in more hazardous situations than other road users. This is reflected by
actuaries when they set insurance rates for different age groups, partly based on their age, sex, and choice of vehicle. Older drivers with slower reactions might be expected to be involved in more collisions, but this has not been the case as they tend to drive less and, apparently, more cautiously. Attempts to impose traffic policies can be complicated by local circumstances and driver behavior. In 1969 Leeming warned that there is a balance to be struck when "improving" the safety of a road. and another argued that compulsory seat belt laws have not been accompanied by a clearly attributed fall in overall fatalities. Most claims of risk compensation offsetting the effects of vehicle regulation and belt use laws have been discredited by research using more refined data. Some crashes are intended;
staged crashes, for example, involve at least one party who hopes to crash a vehicle in order to submit lucrative claims to an insurance company. In the United States during the 1990s, criminals recruited Latin American immigrants to deliberately crash cars, usually by cutting in front of another car and slamming on the brakes. It was an illegal and risky job, and they were typically paid only $100. Jose Luis Lopez Perez, a staged crash driver, died after one such maneuver, leading to an investigation that uncovered the increasing frequency of this type of crash.
Motor vehicle speed in
Helsinki, Finland, on August 25, 2006, at around 13:00 local time. The incident caused
traffic congestion. The U.S. Department of Transportation's
Federal Highway Administration reviewed research on traffic speed in 1998. The summary says that: • The evidence shows the risk of having a crash is increased both for vehicles traveling slower than the average speed and for those traveling above the average speed. • The risk of being injured increases exponentially with speeds much faster than the median speed. • The severity/lethality of a crash depends on the vehicle speed change at impact. • There is limited evidence suggesting lower speed limits result in lower speeds on a system-wide basis. • Most crashes related to speed involve speed too fast for the conditions. • More research is needed to determine the effectiveness of
traffic calming. In the U.S. in 2018, 9,378 people were killed in motor vehicle crashes involving at least one speeding driver, which accounted for 26% of all traffic-related deaths for the year. In Michigan in 2019, excessive speed was a factor in 18.8% of the fatalities that resulted from fatal motor vehicle crashes and in 15.6% of the suspected serious injuries resulting from crashes. The Road and Traffic Authority (RTA) of the Australian state of
New South Wales (NSW) asserts speeding (traveling too fast for the prevailing conditions or above the posted
speed limit) is a factor in about 40 percent of road deaths. The RTA also says speeding increases the risk of a crash and its severity. The contributory factor report in the
official British road casualty statistics shows for 2006, that "exceeding the speed limit" was a contributory factor in 5% of all casualty crashes (14% of all fatal crashes), and "traveling too fast for conditions" was a contributory factor in 11% of all casualty crashes (18% of all fatal crashes). In France, in 2018, the speed limit was reduced from 90 km/h to 80 km/h on a large part of the local outside built-up area road network in the sole aim of reducing the number of road fatalities.
Assured clear distance ahead A common cause of collisions is driving faster than one can stop within their
field of vision. Such practice is illegal and is particularly responsible for an increase in fatalities at night – when it occurs most.
Driver impairment Driver impairment describes factors that prevent the driver from driving at their normal level of skill. Common impairments include: ;Alcohol :According to the
Government of Canada, coroner reports from 2008 suggested almost 40% of fatally injured drivers consumed some quantity of
alcohol before the collision. ;Physical impairment :Poor
eyesight and/or
physical impairment, with many jurisdictions setting simple sight tests and/or requiring appropriate vehicle modifications before being allowed to drive. ;Youth :Insurance statistics demonstrate a notably higher incidence of collisions and fatalities among drivers aged in their teens or early twenties, with insurance rates reflecting this data. These drivers have the highest incidence of both collisions and fatalities among all driver age groups, a fact that was observed well before the advent of mobile phones.Females in this age group exhibit somewhat lower collision and fatality rates than males but still register well above the median for drivers of all ages. Also within this group, the highest collision incidence rate occurs within the first year of licensed driving. For this reason, many US states have enacted a
zero-tolerance policy wherein receiving a moving violation within the first six months to one year of obtaining a license results in automatic license suspension. South Dakota is the only state that allows fourteen-year-olds to obtain drivers' licenses. ;Old age :
Old age, with some jurisdictions requiring driver retesting for reaction speed and eyesight after a certain age. ;Sleep deprivation Various factors such as fatigue or sleep deprivation might increase the risk, or the number of hours of driving might increase the risk of an incident. 41% of drivers self-report having fallen asleep at the wheel. It is estimated that 15% of fatal crashes involve drowsiness (10% of daytime crashes, and 24% of nighttime crashes). Work factors can increase the risk of drowsy driving such as long or irregular hours or driving at night.Cell phone use is an increasingly significant problem on the roads and the U.S.
National Safety Council compiled more than 30 studies postulating that hands-free is not a safer option because the brain remains distracted by the conversation and cannot focus solely on the task of driving.
Combinations Several conditions can combine to create a more dangerous situation, for example, low doses of alcohol and
cannabis have a more severe effect on driving performance than either in isolation. Taking recommended doses of several drugs together, which individually do not cause impairment, may cause drowsiness. This could be more pronounced in an elderly person whose renal function is less efficient than a younger person's.
Road design ,
median barriers, or other physical objects can help reduce the consequences of a collision, or minimize damage. A 1985 US study showed that about 34% of serious crashes had contributing factors related to the roadway or its environment. Most of these crashes also involved a human factor. Research has shown that careful design and maintenance, with well-designed intersections, road surfaces, visibility and traffic control devices, can result in significant improvements in collision rates. Individual roads also have widely differing performance in the event of an impact. In Europe, there are now
EuroRAP tests that indicate how "self-explaining" and forgiving a particular road and its roadside would be in the event of a major incident. In the UK, research has shown that investment in a safe road infrastructure program could yield a reduction in road deaths, saving as much as £6 billion per year. A consortium of 13 major road safety stakeholders had formed the
Campaign for Safe Road Design, which was calling on the
UK Government to make safe road design a national transport priority.
Vehicle design and maintenance involved in a
rollover crash ;Seat belts Research has shown that, across all collision types, it is less likely that
seat belts were worn in collisions involving death or serious injury, rather than light injury; wearing a seat belt reduces the risk of death by about 45 percent.
Seat belt use is controversial, with notable critics such as
Professor John Adams suggesting that their use may lead to a net increase in road casualties due to a phenomenon known as
risk compensation. Observation of driver behaviors before and after seat belt laws does not support the risk compensation hypothesis. Several driving behaviors were observed on the road before and after the belt use law was enforced in Newfoundland, and in Nova Scotia during the same period without a law. Belt use increased from 16 percent to 77 percent in Newfoundland and remained virtually unchanged in
Nova Scotia. Four driver behaviors (speed, stopping at intersections when the control light was amber, turning left in front of oncoming traffic, and gaps in following distance) were measured at various sites before and after the law. Changes in these behaviors in Newfoundland were similar to those in Nova Scotia, except that drivers in Newfoundland drove slower on expressways after the law, contrary to the risk compensation theory. ;Maintenance A well-designed and well-maintained vehicle, with good brakes, tires and well-adjusted suspension will be more controllable in an emergency and thus be better equipped to avoid collisions. Some mandatory
vehicle inspection schemes include tests for some aspects of
roadworthiness, such as the UK's
MOT test or German
TÜV conformance inspection. The design of
vehicles has also evolved to improve protection after collision, both for vehicle occupants and for those outside of the vehicle. Much of this work was led by automotive industry competition and technological innovation, leading to measures such as
Saab's safety cage and reinforced roof pillars of 1946, Ford's 1956
Lifeguard safety package, and Saab and
Volvo's introduction of standard fit
seatbelts in 1959. Other initiatives were accelerated as a reaction to consumer pressure, after publications such as
Ralph Nader's 1965 book
Unsafe at Any Speed accused motor manufacturers of indifference to safety. In the early 1970s,
British Leyland started an intensive program of vehicle safety research, producing a number of prototype
experimental safety vehicles demonstrating various innovations for occupant and pedestrian protection such as
airbags,
anti-lock brakes, impact-absorbing side-panels, front and rear head restraints, run-flat tires, smooth and deformable front-ends, impact-absorbing bumpers, and retractable headlamps. The design has also been influenced by government legislation, such as the
Euro NCAP impact test. Common features designed to improve safety include thicker pillars, safety glass, interiors with no sharp edges,
stronger bodies, other active or passive safety features, and smooth exteriors to reduce the consequences of an impact on pedestrians. The UK
Department for Transport publish road casualty statistics for each type of collision and vehicle through its
Road Casualties Great Britain report. These statistics show a ten-to-one ratio of in-vehicle fatalities between types of cars. In most cars, occupants have a 2–8% chance of death in a two-car collision. ;Center of gravity Some crash types tend to have more serious consequences.
Rollovers have become more common in recent years, perhaps due to the increased popularity of taller
SUVs,
people carriers, and
minivans, which have a higher
center of gravity than standard passenger cars. Rollovers can be fatal, especially if the occupants are ejected because they were not wearing
seat belts (83% of ejections during rollovers were fatal when the driver did not wear a seat belt, compared to 25% when they did). After retrofitting these systems to its models in 1999–2000, Mercedes saw its models involved in fewer crashes. Now, about 40% of new US vehicles, mainly the SUVs, vans and pickup trucks that are more susceptible to rollover, are being produced with a lower
center of gravity and enhanced suspension with
stability control linked to its
anti-lock braking system to reduce the risk of rollover and meet US federal requirements that mandate anti-rollover technology by September 2011. ;Motorcycles Motorcyclists and pillion-riders have little protection other than their
clothing and helmets.This difference is reflected in the casualty statistics, where they are more than twice as likely to suffer severely after a collision. In 2005, there were 198,735 road crashes with 271,017 reported casualties on roads in Great Britain. This included 3,201 deaths (1.1%) and 28,954 serious injuries (10.7%) overall. Of these casualties 178,302 (66%) were car users and 24,824 (9%) were motorcyclists, of whom 569 were killed (2.3%) and 5,939 seriously injured (24%).Motorcyclists in urban areas are also more prone to deadly accidents because urban areas are more populous and roads are more crowded with vehicles.
Sociological factors Studies in
United States have shown that poor people have a greater risk of dying in a car crash than people who are well-off. Car deaths are also higher in poorer states.There is also data that suggests that there is no significant association between injurity severity and socioeconomic status, despite the fact that people with low socioeconomic status are more at risk to be hospitalized for injuries sustained in traffic collisions. Similar studies in
France or
Israel have shown the same results. This may be due to working-class people having less access to secure equipment in cars, having older cars which are less protected against crash, and needing to cover more distance to go to work each day.
COVID-19 lockdown impact While the advent of the COVID lockdown meant a decrease in road traffic in the United States, the rates of incidents, speeding, and traffic fatalities rose in 2020 and 2021 (rate as measured against vehicle miles traveled). The traffic fatality rate jumped to 1.25 per 100 million vehicle miles traveled, up from 1.06 during the same period in 2019. Reasons cited for the increases are greater speeds, not wearing seatbelts, and driving while impaired. Preliminary data also show that even as traffic levels returned to normal after the onset of COVID in March–April 2020, drivers continued to drive at excessive speeds. A 2020 study conducted by INRIX, private company that analyzes traffic patters, behaviors and congestion, showed that as traffic levels returned to normal during the three-month period August to October 2020, growth in collisions (57%), outpaced the growth in miles traveled (22%) resulting in a higher than normal collision rate during this period. It also reported that the number of vehicles on the road dropped by 75%, which suggests the rate (incidents per vehicle-mile) increased.
Other Other possibly hazardous factors that may alter a driver's soundness on the road include: •
Irritability • Following specifically distinct rules too bureaucratically, inflexibly or rigidly when unique circumstances might suggest otherwise • Sudden swerving into somebody's blind spot without first clearly making oneself visible through the
wing mirror • Unfamiliarity with one's
dashboard features,
center console or other interior handling devices after a recent car purchase • Lack of
visibility due to
windshield design, dense
fog or sun glare •
People-watching. • Traffic safety culture, a variety of aspects of
safety culture could impact on the number of crashes. ==Prevention==