annual change traffic

Road traffic safety

Road traffic safety aims to reduce the harm (deaths, injuries, and property damage) resulting from crashes of road vehicles. Harm from road traffic crashes is greater than that from all other transportation modes (air, sea, space, off-terrain, etc.) combined.

Road traffic safety deals exclusively with road traffic crashes – how to reduce their number and their consequences. A road traffic crash is an event involving a road vehicle that results in harm. For reasons of clear data collection, only harm involving a road vehicle is included. A person tripping with fatal consequences on a public road is not included as a road-traffic fatality. To be counted a pedestrian fatality, the victim must be struck by a road vehicle.

Background

Road traffic crashes are one of the world’s largest public health and injury prevention problems. The problem is all the more acute because the victims are overwhelmingly healthy prior to their crashes. According to the World Health Organization more than a million people are killed on the world’s roads each year..

Types of harm

Fatality

Conceptually, the clearest type of harm in a road traffic crash is death – or a fatality. However, the definition of a road-traffic fatality is far more complicated than a casual thought might indicate, and involves many essentially arbitrary criteria. In the United States, for example, the definition used in the Fatality Analysis Reporting System (FARS) run by the NHTSA is a person who dies within 30 days of a crash on a US public road involving a vehicle with an engine, the death being the result of the crash. In America therefore, if a driver has a non-fatal heart attack that leads to a road-traffic crash that causes death, that is a road-traffic fatality. However, if the heart attack causes death prior to the crash, then that is not a road-traffic fatality.

To make matters more complex the definition of Road Accident Fatality can differ in the same country during different years. For example, fatality is defined in France as a person who dies in the 6 days (pre 2005) after the accident; in the 30 days (post 2005) after the accident..

Injuries

It is highly uncertain exactly how many road traffic crash injuries occur in the world. Whether an injury is reported may depend upon compensation and medical procedures as well as on the amount of harm.

Property Damage

Data for property damage crashes is even more uncertain than for injuries. In some jurisdictions the criterion for reporting is damage exceeding some monetary amount specified by statute. Because of inflation, this requirement may include more and more minor crashes as time passes, until the amount is abruptly changed, thereby reducing the reported number of crashes. Drivers generally report single-vehicle property damage crashes only if they see some benefit in reporting them, regardless of legal obligations.

Crash Rates

The safety performance of roadways are almost always reported as rates. That is, some measure of harm (deaths, injuries, or property damage) divided by some indicator of exposure to the risk of this harm. Simple counts are almost never used. The annual count of fatalities is a rate, namely, the number of fatalities per year. Common rates related to road traffic fatalities include the number of deaths per capita, per registered vehicle, per licensed driver, or per vehicle mile traveled.

There is no one rate that is superior to others in any general sense. The rate to be selected depends on the question being asked – and often also on what data are available. What is important is to specify exactly what rate is measured and how it relates to the problem being addressed.

Defining the problem

The standard measures used in assessing road safety interventions are fatalities and Killed or Seriously Injured (KSI) rates, usually per billion (10⁹) passenger kilometres. In the United States, crashes per million vehicle miles is typically used for road safety.

Speed is a key goal of modern road design, but impact speed affects the severity of injury to both occupants and pedestrians. For occupants, Joksch (1993) found the probability of death for drivers in multi-vehicle accidents increased as the fourth power of impact speed (often referred to by the mathematical term δv ("delta V"), meaning change in velocity). Injuries are caused by sudden, severe acceleration (or deceleration), this is difficult to measure. However, crash reconstruction techniques can be used to estimate vehicle speeds before a crash. Therefore, the change in speed is used as a surrogate for acceleration.

Interventions

Interventions take many forms. Contributing factors to highway crashes may be related to the driver (such as driver error, illness or fatigue), the vehicle (brake, steering, or throttle failures) or the road itself (lack of sight distance, poor roadside clear zones, etc). Interventions may seek to reduce or compensate for these factors, or reduce the severity of crashes that do occur. A comprehensive outline of interventions areas can be seen in Management systems for road safety.

Road design

On neighborhood roads where many vulnerable road users, such as pedestrians and bicyclists can be found, traffic calming can be a tool for road safety. Shared space schemes, which rely on human instincts and interactions, such as eye contact, for their effectiveness, and are characterised by the removal of traditional traffic signals and signs, and even by the removal of the distinction between carriageway (roadway) and footway (sidewalk), are also becoming increasingly popular. Both approaches can be shown to be effective.

Outside neighborhood roads, design features are added to increase motorized safety and mobility. These features come at increasing costs; costs which include monetary amounts, decreased or discouraged usage by non-motorized travelers, as well as aesthetics. Benefits include a broader spectrum of occupational, cultural and entertainment options than enjoyed by more travel-limited generations.

At the other end of the spectrum from neighborhood roads are motorways, which may be called freeways, limited access highways, Autobahnen, Interstates or other national names. Motorways have the best engineered road features, limited access and minimise opportunities for conflict so are typically the safest roads per mile travelled and offer better fuel economy despite higher average speeds.

Road Design Features

Better motorways are banked on curves in order to reduce the need for tire-traction and increase stability for vehicles with high centers of gravity. Most roads are cambered (crowned), that is, made so that they have rounded surfaces, to reduce standing water and ice, primarily to prevent frost damage but also increasing traction in poor weather. Some sections of road are now surfaced with porous bitumen to enhance drainage; this is particularly done on bends.

Modern safety barriers are designed to absorb impact energy and minimize the risk to the occupants of cars, and bystanders. For example, most side rails are now anchored to the ground, so that they cannot skewer a passenger compartment, and most light poles are designed to break at the base rather than violently stop a car that hits them. Some road fixtures such as road signs and fire hydrants are designed to collapse on impact. Highway authorities have also removed trees in the vicinity of roads; while the idea of "dangerous trees" has attracted a certain amount of skepticism, unforgiving objects such as trees can cause severe damage and injury to any errant road users.

An example of the importance of roadside clear zones can be found on the Isle of Man TT motorcycle race course. It is much more dangerous than Silverstone because of the lack of runout. When a rider falls off at Silverstone he slides along slowly losing energy, so minimal injuries. When he falls of in the Manx he impacts with trees and walls. Similarly, a clear zone alongside a freeway or other high speed road can prevent off-road excursions from becoming fixed-object crashes.

The ends of some guard rails on high-speed highways in the United States are protected with impact attenuators, designed to gradually absorb the kinetic energy of a vehicle and slow it more gently before it can strike the end of the guard rail head on, which would be devastating at high speed. Several mechanisms are used to dissipate the kinetic energy. Fitch Barriers, a system of sand-filled barrels, uses momentum transfer from the vehicle to the sand. Many other systems tear or deform steel members to absorb energy and gradually stop the vehicle.

Road hazards and intersections in some areas are now usually marked several times, roughly five, twenty and sixty seconds in advance so that drivers are less likely to attempt violent maneuvers.

Most road signs and pavement marking materials are retro-reflective, incorporating small glass spheres or prisms to more efficiently reflect light from vehicle headlights back to the driver's eyes.

Lane markers in some countries and states are marked with Cat's eyes or Botts dots, bright reflectors that do not fade like paint. Botts dots are not used where it is icy in the winter, because frost and snowplows can break the glue that holds them to the road, although they can be embedded in short, shallow trenches carved in the roadway, as is done in the mountainous regions of California.

In some countries major roads have "tone bands" impressed or cut into the edges of the legal roadway, so that drowsing drivers are awakened by a loud hum as they release the steering and drift off the edge of the road. Tone bands are also referred to as "rumble strips," owing to the sound they create. An alternative method is the use of "Raised Rib" markings, which consists of a continuous line marking with ribs across the line at regular intervals. They were first specially authorised for use on motorways as an edge line marking to separate the edge of the hard shoulder from the main carriageway. The objective of the marking is to achieve improved visual delineation of the carriageway edge in wet conditions at night. It also provides an audible/vibratory warning to vehicle drivers, should they stray from the carriageway, and run onto the marking.

The U.S. has developed a prototype automated roadway, to reduce driver fatigue and increase the carrying capacity of the roadway. Roadside units participating in future Wireless vehicle safety communications networks have been studied.

There is some controversy over the way that the motor lobby has been seen to dominate the road safety agenda. Some road safety activists use the term "road safety" (in quotes) to describe measures such as removal of "dangerous" trees and forced segregation of the vulnerable to the advantage of motorized traffic. Orthodox "road safety" opinion fails to address what Adams describes as the top half of the risk thermostat, the perceptions and attitudes of the road user community.

Motorway

Motorways (called freeways in North America) have the highest design standards for speed, safety and fuel efficiency. Motorways improve safety by:

prohibiting more vulnerable road users
prohibiting slow-moving vehicles, thus reducing speed variation and potential δv for same-direction travel
segregating opposing traffic flows with median dividers or crash barriers, thus reducing potential δv for opposite-direction collisions
separating crossing traffic by replacing intersections with interchanges, thus reducing potential δv into the side, most vulnerable vehicle section (side impacts are also responsible for some of the most serious traumatic brain injuries)
removing roadside obstacles.

Although these roads may experience greater severity than most roads to due higher speeds in the event of a crash, the probability of a crash is reduced by removing interactions (crossing, passing, slower and opposing traffic), and crash severity is reduced by removing massive, fixed objects or surrounding them with energy attenuation devices (e.g. guardrails, wide grassy areas, sand barrels). These mechanisms deliver lower fatalities per vehicle-kilometer of travel than other roadways, as documented in the following table.

Killed per 1 Billion Veh·km (Motorways in 2003)	Killed per 1 Billion Veh·km (Non-Motorways in 2003)	Motorway AADT	Road Travel by Motorway	km/h (mph) Motorway 2003 Speed Limit
5.9	13.4	30,077	23%	130 (80)
9.9	34.3	25,714	11%	130 (80)
3.0	11.9	29,454	25%	130 (80)
1.4	8.3	22,780	10%	120 (75)
4.0	12.8	31,979	21%	130 (80)
3.8	12.4	48,710	31%	130 (80) (advisory)
7.4	11.0	26,730	4%	120 (75)
4.0	11.9	26,152	9%	100 (60)
2.1	11.7	66,734	41%	120 (75)
8.1	18.7	15,643	19%	130 (80)
2.5	9.9	24,183	21%	110 (70)
2.8	11.8	43,641	33%	120 (75)
2.0	9.3	85,536	23%	110 (70)
5.2	10.7	39,634	24%	120 (75)

definition: AADT - average annual daily traffic. The bi-direction traffic count representing an average 24-hour day in a year. Sometimes called "traffic density" although it ignores or assumes a constant number of travel lanes.

source: International Road Traffic and Accident Database (IRTAD) , Risk Values in 2003 and Selected References Values for 2003 -- courtesy of the Bundesanstalt für Straßenwesen, that is, the (German) Federal Highway Research Institute. Travel was computed by dividing the fatality rate by the number of fatalities; AADT by dividing travel by the length of the motorway network. 2003 speed limits were obtained from the Wiki page and verified with other sources.

Motorways are far more expensive and space-consumptive to build than ordinary roads, so are only used as principal arterial routes. In developed nations, motorways bear a significant portion of motorized travel; for example, the United Kingdom's 3533 km of motorways represented less than 1.5% of the United Kingdom's roadways in 2003, but carry 23% of road traffic.

The proportion of traffic borne by motorways is a significant safety factor. For example, even though the United Kingdom had a higher fatality rates on both motorways and non-motorways than Finland, both nations shared the same overall fatality rate in 2003. This result was due to the United Kingdom's higher proportion of motorway travel.

Similarly, the reduction of conflicts with other vehicles on motorways results in smoother traffic flow, reduced collision rates, and reduced fuel consumption compared with stop-and-go traffic on other roadways.

The improved safety and fuel economy of motorways are common justifications for building more motorways. However, the planned capacity of motorways is often exceeded in a shorter timeframe than initially planned, due to the under estimation of the extent of the suppressed demand for road travel. In developing nations, there is significant public debate on the desirability of continued investment in motorways.

Motorways around the world are subject to a broad range of speed limits. Recent experiments with variable speed limits based on automatic measurements of traffic density have delivered both improvements in traffic flow and reduced collision rates, based on principles of turbulent flow analysis.

With effect from January 2005 and based primarily on safety grounds, the UK’s Highways Agency's policy is that all new motorway schemes are to use high containment concrete step barriers in the central reserve. All existing motorways will introduce concrete barriers into the central reserve as part of ongoing upgrades and through replacement as and when these systems have reached the end of their useful life. This change of policy applies only to barriers in the central reserve of high speed roads and not to verge side barriers. Other routes will continue to use steel barriers.

Pavement Design

Poor pavement construction can lead to safety problems. If too much asphalt or bitumenous binder is used in asphalt concrete, the binder can 'bleed' or flush' to the surface, leaving a very smooth surface that provides little traction when wet. Certain kinds of stone aggregate become very smooth or polished under the constant wearing action of vehicle tires, again leading to poor wet-weather traction. Either of these problems can increase wet-weather crashes by increasing braking distances or contributing to loss of control. If the pavement is insufficiently sloped or poorly drained, standing water on the surface can also lead to wet-weather crashes.

Rating roads for safety

Since 1999 the EuroRAP initiative has been assessing major roads in Europe with a road protection score. This results in a star rating for roads based on how well its design would protect car occupants from being severely injured or killed if a head-on, run-off, or intersection accident occurs, with 4 stars representing a road with the best survivability features. The scheme states it has highlighted thousands of road sections across Europe where road-users are routinely maimed and killed for want of safety features, sometimes for little more than the cost of safety fencing or the paint required to improve road markings.

There are plans to extend the measurements to rate the probability of an accident for the road. These ratings are being used to inform planning and authorities' targets. For example, in Britain two-thirds of all road deaths in Britain happen on rural roads, which score badly when compared to the high quality motorway network; single carriageways claim 80% of rural deaths and serious injuries, while 40% of rural car occupant casualties are in cars that hit roadside objects, such as trees. Improvements in driver training and safety features for rural roads are hoped to reduce this statistic.

Alternative approaches

For most of the 20th Century, many road authorities believed that traffic should be separated and controlled by class (pedestrian, bicyclist, and motor vehicle driver) in order to combine high speed use of motor vehicles with good traffic safety. Acceptance of this view by the authorities has led to the widespread use of traffic lights, Belisha beacons, pedestrian crossings, and, in some jurisdictions, cycle lanes.

Alternatives to this approach have been suggested, such as those offered by J. S. Dean, a former chairman of the UK Pedestrians' Association: Better roads, better sight lines, fewer bends and blind corners, less traffic, better lighting, better visibility, better weather conditions – all these that are supposed to make for greater safety, in fact, make for greater danger ... because every "nonrestrictive" safety measure, however admirable in itself, is treated by the drivers as an opportunity for more speeding, so that the net amount of danger is increased and the latter state is worse than the first.

In the latter part of the 20th Century, alternative design approaches such as those pioneered in woonerven became more popular. These approaches, recognising that higher traffic speeds made collisions more likely and injuries more severe, sought to reduce traffic speeds in community and housing zones by the use of lower speed limits enforced by the use of special signage and road markings, the introduction of traffic calming measures, and by giving pedestrians priority over motorists.

Beginning in the 1990s, traffic professionals such as Hans Monderman, recognising that even with all the traditional measures of speed limits, traffic calming, pedestrian barriers, road signs and road markings, that safety and congestion problems remained, started another revolution in planning. John Adams argues that traditional traffic engineering measures assumed that motorists were "selfish, stupid, obedient automatons who had to be protected from their own stupidity" and non-motorists were treated as "vulnerable, stupid, obedient automatons who had to be protected from cars – and their own stupidity". Monderman believed that "if you treat drivers like idiots, they act as idiots" and that you should "always assume they have intelligence". He found that trusting drivers to behave was more successful than forcing them to behave. Monderman's new approach was characterised by the redesign of junctions and road layouts, a virtual elimination of kerbs, road signs and signals, the removal of cycling lanes and speed humps, and the introduction of equal priority for all types of road user, with right of way being open to negotiation. This philosophy, under the name "shared space", has since attracted the attention of many authorities around the world. Reported results indicate that the shared space approach leads to significantly reduced traffic speeds, the virtual elimination of road casualties, and a reduction in congestion.

At least one town, Hesselterbrink in the Netherlands has become disillusioned with the way the woonerf principle has become another traffic engineering measure that "entailed preciously little more than signs and uniform standards". They have now encompassed the shared space principles in favour of the woonerf. They are reported to "now know that car drivers should become residents. Eye contact and human interaction are more effective means to achieve and maintain attractive and safe areas than signs and rules".

Motorized vehicles and their drivers

For more information, see article automobile safety

Safety can be improved by reducing the chances of a driver making an error, or by designing vehicles to reduce the severity of crashes that do occur. Safety interventions focusing on motorized vehicles and their drivers include:

Seat belts, including seat belt legislation. Seat belts are now fitted by law in both front and rear of most passenger cars and an increasing number of public transit vehicles.
Airbags
Electronic Stability Control
Safety cages, which protect the driver from intrusion by impacting objects, and crumple zones, which absorb collision energy.
Compulsory training and licensing (although this is often a once-off requirement some countries require periodic retests and others will require drivers convicted of offences to undergo certain training and retests before being allowed back on the roads). (see: traffic psychology)
Restrictions on driving while drunk or impaired by drugs.
Photo enforcement of speed limits.
Restrictions on mobile phone use while on the move.
Compulsory safety testing of vehicles over a certain age.
Compulsory insurance to compensate victims.
Restrictions on commercial vehicle driver hours, and fitting of tachographs.
Crash compatibility, including efforts to match bumper heights and sill heights, moderate differences in vehicle weight, and match crumple zone stiffness.

Some of these interventions have been opposed by car manufacturers (see Unsafe at Any Speed) or by drivers, or by academics who believe that because of the risk compensation effect some of these measures may actually reduce road safety overall.

Employers currently escape, for the most part, the chain of responsibility for their employees' driving on company business. Truck drivers, especially self-employed ones, can be given unrealistic deadlines to meet. There are moves to bring driving for work (both commercial vehicles and, more controversially, private cars driven on company business) under the umbrella of workplace safety legislation. These are strongly resisted as they would place a far greater burden on employers and employees alike: penalties for industrial safety infractions are typically much greater than for negligent motor vehicle use.

Other road users

Pedestrians and Cyclists are among the most vulnerable road users, and in some countries consitute over half of all road deaths. Interventions aimed at improving safety of non-motorised users:

segregated facilities such as cycle lanes, underpasses and overbridges
physical separation of segregated facilities
pedestrian barriers to prevent pedestrians crossing at junctions
limiting pedestrian access to highways
bicycle helmet promotion and compulsion
traffic awareness campaigns such as the "one false move" campaign documented by Hillman et al.
pedestrian crossings, which are seen as restricting the number of points at which a road may be crossed and often requiring detours.
traffic calming and speed humps
shared space schemes giving ownership of the road space and equal priority to all road users, regardless of mode of use
reduced urban speed limits
rigorous speed limit enforcement by automated means such as speed cameras

The introduction of road safety improvements targetted at vulnerable road users could be significant. In Costa Rica, for example, 57% of road deaths are pedestrians, however a partnership between AACR, Cosevi, MOPT and iRAP has proposed the construction of 190km of pedestrian footpaths and 170 pedestrian crossings has been proposed which could save over 9000 fatal or serious injuries over 20 years .

Road safety advertising

Road traffic authorities around the world have mounted advertising campaigns to convince drivers to operate vehicles safely. Examples include:

Criticisms

Non-motorised lobby

Pedestrians' advocates, environmental groups and related organisations such as RoadPeace have been strongly critical of what they see as moves to solve the problem of danger posed to vulnerable road users by motor traffic through increasing restrictions on vulnerable road users, an approach which they believe both blames the victim and fails to address the problem at source. This is discussed in detail by Dr Robert Davis in the book Death on the Streets: Cars and the mythology of road safety, and the core problem is also addressed in books by Professor John Adams, Mayer Hillman and others.

It is argued by some that the problem of road safety is largely being stated in the wrong terms because most road safety measures are designed to increase the safety of drivers, but many road traffic casualties are not drivers (in the UK only 40% of casualties are drivers), and those measures which increase driver safety may, perversely, increase the risk to these others, through risk compensation.

The core elements of the thesis are:

that vulnerable road users are marginalised by the "road safety" establishment
that "road safety" interventions are often centred around reducing the severity of results from dangerous behaviours, rather than reducing the dangerous behaviours themselves
that improved "road safety" has often been achieved by making the roads so hostile that those most likely to be injured cannot use them at all
that the increasing "safety" of cars and roads is often counteracted wholly or in part by driver responses (risk compensation).

Pedestrians in particular are often reluctant to use segregated facilities which involve them in extra distance, extra effort (e.g. overbridges) or perceived extra risk (underpasses, often perceived as a haunt of muggers). Pedestrians' advocates question the equitability of reducing the danger posed to pedestrians by car drivers, through mechanisms which place the primary burden on the victims. Recently (2007) criticism of the campaign "Make Roads Safe" was put forward on similar grounds.

Case study: UK pedestrian safety

The UK claims among the best pedestrian safety records in Europe, as measured in pedestrian KSI per head of population, comparing similarly to Australia in 2006. But it has been noted that this value would also be low if the roads were sufficiently dangerous as to deter pedestrians from using them at all. One way of testing this hypothesis would be to compare rates for those whose transport options are most limited, the elderly and children. Hillman and others have done this and found that:

Britain's child pedestrian safety record is worse than the average for Europe, in contrast to the better than average all-ages figure (Department for Transport)
Children's independent mobility is increasingly curtailed, with fear of traffic being cited as a dominant cause (Hillman, Adams, Whitelegg)
Distances walked have declined more than in other European countries
Similar (though less well-defined) observations can be made regarding the elderly

These conclusions are reported in Hillman, Adams and Whitelegg's One False Move (Policy Studies Insititue, ISBN ).

Motorised lobby

Driver's organisations and road safety campaigning organisations such as the Association of British Drivers and Safe Speed in the UK, and the National Motorists Association in the USA and Canada argue that the strict enforcement of speed limits does not necessarily result in safer driving, and may even have a negative effect on road safety in general.

The Association of British Drivers also argues that speed humps result in increased air pollution, increased noise pollution, and even unnecessary vehicle damage.

United Nations response

Owing to the global and massive scale of the issue, with predictions that by 2020 road traffic deaths and injuries will exceed HIV/AIDS as a burden of death and disability, the United Nations and its subsidiary bodies have passed resolutions and held conferences on the issue. The first United Nations General Assembly resolution and debate was in 2003 The World Day of Remembrance for Road Traffic Victims was declared in 2005. In 2009 the first high level ministerial conference on road safety will be held in Moscow.

Academic resources

Accident Analysis & Prevention ISSN: , Elsevier
International Journal of Injury Control and Safety Promotion, ISSN: (electronic) (paper), Taylor & Francis
Traffic Injury Prevention, ISSN: 1538-957X (electronic) (paper), Taylor & Francis