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 young and 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 [1].

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) [2] 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.

Injuries

How many road-traffic crash injuries occur in the world? The answer to this question is highly uncertain. 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

File:Highway4safetycorridor.jpg

One method is to post special safety signage on the most dangerous highways.

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 (both young and old) 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.

Most street furniture is now 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 street furniture is designed to collapse on impact. Highways 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 loosing 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
year 2003			Motorway	Motorway Usage	Maximum Motorway
Country	Motorways	Non-Motorways	AADT	(% of Road Travel)	Speed Limit in 2003 in km/h (mph)
Austria	5.9	13.4	30,077	23%	130 (80)
Czech Republic	9.9	34.3	25,714	11%	130 (80)
Denmark	3.0	11.9	29,454	25%	130 (80)
Finland	1.4	8.3	22,780	10%	120 (75)
France	4.0	12.8	31,979	21%	130 (80)
Germany	3.8	12.4	48,710	31%	130 (80) (advisory)
Ireland	7.4	11.0	26,730	4%	120 (75)
Japan	4.0	11.9	26,152	9%	100 (60)
The Netherlands	2.1	11.7	66,734	41%	120 (75)
Slovenia	8.1	18.7	15,643	19%	130 (80)
Sweden	2.5	9.9	24,183	21%	110 (70)
Switzerland	2.8	11.8	43,641	33%	120 (75)
United Kingdom	2.0	9.3	85,536	23%	110 (70)
United States	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) [3], 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 insufficently sloped or poorly drained, standing water on the surface can also lead to wet-weather crashes.

Motorized vehicles and their drivers

For more information, see article automobile safety

Crash simulator : promoting safety through education

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

Interventions aimed at improving safety of non-motorised users:

segregated facilities such as cycle lanes, underpasses and overbridges
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

Road safety advertising

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

Speeding. No one thinks big of you. (New South Wales, Australia, 2007)

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 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. 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. These claims are not supported by the preponderance of peer-reviewed evidence.

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

Academic resources

Accident Analysis & Prevention ISSN: , Elsevier
International Journal of Injury Control and Safety Promotion, ISSN: [4] (electronic) (paper), Taylor & Francis
Traffic Injury Prevention, ISSN: 1538-957X (electronic) (paper), Taylor & Francis

Notes

External links

Human Error in Road Accidents
Intelligent Communicating Transport Systems
Global Road Safety Partnership
IRTAD - International Road Traffic and Accident Database
An Indian Road Safety and Accident Data Management System
INTRAS - Research Institute on Traffic and Road Safety of the University of Valencia, Spain
Road Safety - Transport - European Commission
UK occupational road safety web site
Charles Adler, Jr. Collection, ca. Archives Center, National Museum of American History, Smithsonian Institution.
The Flicker Fusion Factor: Why we can't drive safely at high speed.
Highway Rescue Project in India.