A 1985 study by R. Kumar, using British and American crash reports as data, suggested 57% of crashes were due solely to driver factors, 27% to combined roadway and driver factors, 6% to 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 all factors 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. Intent is also a factor, a vehicle-ramming attack for a example happens largely due to a driver choosing to cause a traffic collision. 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. 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 10 times. An RAC survey of British drivers found that most^[quantify] thought they were better than average drivers; a contradictory result showing overconfidence in their abilities.^{[citation needed]} 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:

• controlling a car including a good awareness of the cars size and capabilities
• reading and reacting to road conditions, weather, road signs and the environment
• alertness, reading and anticipating the behavior of other drivers.

Although proficiency in these skills is taught and tested as part of the driving exam, a good driver can still be at a high risk of crashing because:

...the feeling of being confident in more and more challenging situations is experienced as evidence of driving ability, and that proven ability reinforces the feelings of confidence. Confidence feeds itself and grows unchecked until something happens – a near-miss or an accident.

An AXA survey concluded Irish drivers are very safety-conscious relative to other European drivers. However, this does not translate to significantly lower crash rates in Ireland. 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 drivers 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: Conversely, a location that does not look dangerous may have a high crash frequency. This is, in part, because if drivers perceive a location as hazardous, they take more care. Collisions may be more likely to happen when hazardous road or traffic conditions are not obvious at a glance, or where the conditions are too complicated for the limited human machine to perceive and react in the time and distance available. High incidence of crashes is not indicative of high injury risk. Crashes are common in areas of high vehicle congestion but fatal crashes occur disproportionately on rural roads at night when traffic is relatively light. This phenomenon has been observed in risk compensation research, where the predicted reductions in collision rates have not occurred after legislative or technical changes. One study observed that the introduction of improved brakes resulted in more aggressive driving 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 has been discredited by research using more refined data. In the 1990s, Hans Mondermans studies of driver behavior led him to the realization that signs and regulations had an adverse effect on a drivers ability to interact safely with other road users. Monderman developed shared space principles, rooted in the principles of the woonerven of the 1970s. He concluded that the removal of highway clutter, while allowing drivers and other road users to mingle with equal priority, could help drivers recognize environmental clues. They relied on their cognitive skills alone, reducing traffic speeds radically and resulting in lower levels of road casualties and lower levels of congestion. 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 USA in the 1990s, criminals recruited Latin 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

The U.S. Department of Transportations Federal Highway Administration review research on traffic speed in 1998. The summary says:

• 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.

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 say speeding increases the risk of a crash and its severity. On another web page, the RTA qualify their claims by referring to one specific piece of research from 1997, and writes research has shown that the risk of a crash causing death or injury increases rapidly, even with small increases above an appropriately set speed limit. The contributory factor report in the official British road casualty statistics show for 2006, that exceeding 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).

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.^[32]

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. No US state allows fourteen year-olds to obtain drivers’ licenses any longer.

Old age

Old age, with some jurisdictions requiring driver retesting for reaction speed and eyesight after a certain age.

Sleep deprivation

Fatigue

Drug use

Including some prescription drugs, over the counter drugs (notably antihistamines, opioids and muscarinic antagonists), and illegal drugs.

Distraction

Research suggests that the drivers attention is affected by distracting sounds such as conversations and operating a mobile phone while driving. Many jurisdictions now restrict or outlaw the use of some types of phone within the car. Recent research conducted by British scientists suggests that music can also have an effect; classical music is considered to be calming, yet too much could relax the driver to a condition of distraction. On the other hand, hard rock may encourage the driver to step on the acceleration pedal, thus creating a potentially dangerous situation on the road. Cell phone use is an increasingly significant problem on the roads.^{[citation needed]} 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 of factors

Several conditions can combine to create a much worse situation, for example:

• Combining low doses of alcohol and cannabis has a more severe effect on driving performance than either cannabis or alcohol in isolation, or
• Taking recommended doses of several drugs together, which individually do not cause impairment, may combine to bring on drowsiness or other impairment. This could be more pronounced in an elderly person whose renal function is less efficient than a younger persons.

Thus there are situations when a person may be impaired, but still legally allowed to drive, and becomes a potential hazard to themselves and other road users. Pedestrians or cyclists are affected in the same way and can similarly jeopardize themselves or others when on the road.

Road design

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. The road or environmental factor was either noted as making a significant contribution to the circumstances of the crash, or did not allow room to recover. In these circumstances it is frequently the driver who is blamed rather than the road; those reporting the collisions have a tendency to overlook the human factors involved, such as the subtleties of design and maintenance that a driver could fail to observe or inadequately compensate for. 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 ⅓ reduction in road deaths, saving as much as £6 billion per year. A consortium of 13 major road safety stakeholders have formed the Campaign for Safe Road Design, which is calling on the UK Government to make safe road design a national transport priority.

Vehicle design and maintenance

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.However, actual observation of driver behaviors before and after seat belt laws does not support the risk compensation hypothesis. Several important 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 UKs 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 Saabs safety cage and reinforced roof pillars of 1946, Ford´s 1956 Lifeguard safety package, and Saab and Volvos introduction of standard fit seatbelts in 1959. Other initiatives were accelerated as a reaction to consumer pressure, after publications such as Ralph Naders 1965 book Unsafe at Any Speed accused motor manufacturers of indifference towards safety. In the early 1970s British Leyland started an intensive programme of vehicle safety research, producing a number of prototype experimental safety vehicles demonstrating various innovations for occupant and pedestrian protection such as air bags, 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. 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 with 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 car. 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 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 a new design of Mercedes Benz notoriously failed a moose test (sudden swerving to avoid an obstacle), some manufacturers enhanced suspension using stability control linked to an anti-lock braking system to reduce the likelihood of rollover. 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 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%).

Other

Other possibly hazardous factors that may alter a drivers soundness on the road includes:

• Irritability,
• Following specifically distinct rules too bureaucratically, inflexibly or rigidly when unique circumstances might suggest otherwise
• Sudden swerving into somebodys blind spot without first clearly making oneself visible through the wing mirror
• Unfamiliarity with ones dashboard features, center console or other interior handling devices after a recent car purchase
• Lack of visibility due to windshield design or sun glare
• Distraction by scenery, a sexually attractive person or sexually suggestive advertising