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This is a background article. See : Psychological research into highway safety
Highway safety orRoad-traffic safety aims to reduce the harm (deaths, injuries, and property damage) resulting from crashes of road vehicles traveling on public roads. Harm from road-traffic crashes is greater than that from all other transportation modes (air, sea, space, off-terrain, etc.) combined.
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 .
Definition of road–traffic crashEdit
Because most of the world’s billion drivers have opinions about this subject, it is imperative to separate what scientific investigation has discovered from the rich mix of available material. A clear understanding requires defining what we are talking about.
Road-traffic safety deals exclusively with road-traffic crashes – how to reduce their number and their consequences. A road-traffic crash is an event starting on a public road 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.
The word “accident” began to disappear from professional literature in the late 1960s as more science was focused on the problem. It should not be used for reasons published in the worlds most prestigious peer-reviewed journals and by the preeminent contributors to the science of the subject. For example, Haddon W Jr. Am J Public Health. 1968; 58: 1431-1438; Doege TC. New Engl J Med. 1978; 298: 509-510; Langley JD. Accid Anal Prev. 1988; 20: 1-8; Evans L. Brit Med J. 1993; 307: 1438-1439; Pless B, Davis RM. Brit Med J. 2001; 322: 1321-1322.
The word accident conveys a sense that the losses are due exclusively to fate. Perhaps this is what gives accident its most potent appeal – the sense that it exonerates participants from responsibility. Accident also conveys a sense that losses are devoid of predictability. Yet the purpose of studying safety is to examine factors that influence crashes. Some crashes are purposeful acts for which the term accident would be inappropriate even in popular use. At least a few percent (perhaps as much as 5%) of driver fatalities are suicides, while a lesser number are homicides. Very few of the deaths that are suicides or homicides can be identified as such. When a severely damaged vehicle with a dead driver is observed, saying the driver was killed in a traffic crash describes in simple terms what is observed; calling it an accident implies you know more than you do.
Types of harmEdit
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. If a victim dies many days after a crash, a difficult judgment may be required to decide whether it is a road-traffic fatality. For example, a frail person may die from pneumonia during hospitalization to treat crash trauma. As we all have some chance of dying at any moment, some people die within 30 days of even the most minor crash. While far from perfect, fatality data are by far the most reliable information we have, and much of the solid findings in traffic safety relate to fatalities. The case study can better show the results.
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. While the definition and determination of fatalities are difficult, injuries involve much more complexity.
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.
Crashes resulting in property damage vastly outnumber crashes resulting in minor injuries, which vastly outnumber crashes resulting in major injuries, which vastly outnumber crashes resulting in fatality. Fatality is often referred to as a fatal injury. Most analyses focus on fatalities, injuries (specified as including or not including fatalities) and property damage (specified as including or not including injuries – nearly all crashes damage property).
The total annual cost of property damage ($60 billion) is greater than the fatality cost ($41 billion) for the US (all harm converted to dollar equivalent), reflecting how vastly more minor crashes occur. The total cost of road-traffic crashes to the US in 2000 was $231 billion (Blincoe LJ et al. The economic impact of motor vehicle crashes, 2000. Report DOT HS 809 446. Washington, DC: National Highway Traffic Safety Administration, US Department of Transportation; May 2002).
All measures are rates Edit
The quantities that can be measured in road-traffic safety are nearly always 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.
More details on all the above topics in Traffic Safety (book).
Road-traffic crashes are as old as the roads themselves. Nicolas-Joseph Cugnot crashed his steam-powered "Fardier" against a wall in 1770. Amongst the earliest recorded motor vehicle accident fatalities were Mary Ward who died after being thrown from an experimental steam car on August 31, 1869 in Parsonstown, Ireland, and Bridget Driscoll who was hit by a car on August 17, 1896 in London.
Many of the earliest innovations in road safety are credited to William Phelps Eno, sometimes known as the "father of traffic safety". He is credited with conceiving the stop sign, the traffic circle (roundabout), the one-way street, and many other features of traffic control that are taken for granted today.
The earliest methods for improving road safety included traffic signs and signals, and road markings such as center lines (June McCarroll's idea), as well as compulsory driver testing and licensing.
The foregoing list of early interventions are some examples of the "three E's": Engineering, Education, and Enforcement efforts to overcome human error and imperfect human reliability. Road user error has been recognised as a principal causative factor of collisions from the beginning, since the percentage of crashes directly attributable to animals or mechanical failure is very small. The term "crash" is preferred by authorities rather than the popular term "accidents" so as to also encompass rare but deliberate acts, such as road rage. Generally, crashes appear to be results of the "three I's", that is, inattention, illness, or impairment, rather than malice or terror. Vulnerable road users bear the consequences of the 3 I's, even in the cases when they themselves are inattentive, ill, or impaired rather than a vehicle user being, perhaps, impaired.
Defining the problem Edit
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 accelleration.
Pedestrians travel slowly, so δv is dominated in pedestrian collisions by vehicle speed. Best estimates suggest that 5% of pedestrians who are struck at 20 mph (30 km/h) are killed, 45% at 30 mph (50 km/h) and 85% at 40 mph (65 km/h) (Ashton and Mackay, 1979). On highways there are few pedestrians: same-direction crashes may have a low δv (although this may end up in a high δv if one or both vehicles then hits a stationary object) while opposing-direction crashes will have roughly double the kinetic energy of a single-vehicle crash at the mean free travelling speed, so most highways separate opposing traffic flows.
In the United Kingdom, pedestrians and pedal cyclists accounted for about 45% of KSI in built-up (urban) areas -- compared to 5% of KSI on roads intended solely for motorized traffic. Ongoing safety issues in built-up areas has led in some cases to a surprising reversal of a long-standing strategy: the strategy of segregating motorists from other, more vulnerable road users by the use of footpaths, underpasses, guard rails, etc.
The scale of the problem Edit
Increasing motorisation has resulted in a corresponding growth in crashes and it is currently accepted that in most OECD countries the cost of road traffic collisions amounts to about two per cent of their Gross domestic product (GDP). In developing countries, these losses can be greater than the amount received in international aid and loans, a fact that has prompted the World Bank and the Asian Development Bank to include activities in this field as one of its priorities. In terms of fatalities, the worldwide estimation was 800,000 per year in 1999, forecast to grow to between 1.1 and 1.2 million by 2010 and to between 1.3 and 1.4 million by the year 2020. (Silcock, 2003). It has been estimated that cars have killed more people since their invention than all wars in the same period (including both World Wars).
Casualty rates vary widely from country to country, for reasons which are only imperfectly understood, although Smeed's law has been advanced as a partial explanation.
|KILLED per 1 BILLION (109) Veh·km||year 2003||KILLED per 1 BILLION Veh·km||year 2003|
|Country (alphabetically)||Rate||Country (re-ordered by rate)||Rate|
|Germany||9.7||The United States||9.4|
|New Zealand||12.4||New Zealand||12.4|
|United Kingdom||7.6||Czech Republic||31.7|
|The United States||9.4||Slovak Republic||46.9|
source: International Road Traffic and Accident Database (IRTAD); all countries listed with overall fatality rates.
|Disease or Injury||Disease or Injury|
|1||Lower respiratory infections||1||Ischaemic heart disease|
|2||Diarrhoeal diseases||2||Unipolar major depression|
|3||Perinatal conditions||3||Road traffic injuries|
|4||Unipolar major depression||4||Cerebrovascular disease|
|5||Ischaemic heart disease||5||Chronic obstructive pulmonary disease|
|6||Cerebrovascular disease||6||Lower respiratory infections|
|9||Road traffic injuries||9||Diarrhoeal diseases|
In order to build a ranking, epidemiologist use estimated DALYs (disability-adjusted life years) lost as the measure of the burden of disease.
As can be seen, road traffic injuries are a growing health problem, and for year 2020 it is expected that will come to the third position. This is partly due to improvements in medicine reducing deaths from other causes but largely due to the steady increase in motorisation around the world, reflecting the greater severity of motor traffic versus other causes of injury. In the UK, for example, road traffic injuries are responsible for one in ten child hospital admissions but over half of all injury fatalities (2002 figures).
The scale of road casualties is also a concern for public health because it deters active travel (walking, cycling, etc.), and journeys deterred from these modes themselves become part of the problem.
Interventions take many forms.
Road design Edit
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 Edit
Better highways 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.
The ends of some guard rails on high-speed highways in the United States are protected with Fitch Barriers, a system of sand-filled barrels 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.
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 signs and road line paint are retro-reflective, incorporating small glass spheres to reflect headlights more efficiently.
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.
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.
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 probably 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)|
|Czech Republic||9.9||34.3||25,714||11%||130 (80)|
|Germany||3.8||12.4||48,710||31%||130 (80) (advisory)|
|The Netherlands||2.1||11.7||66,734||41%||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), 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.
The surfaces of all asphalt roads constructed today that are made from petroleum based bitumen have a tendency of becoming greasy (slippery) under wet weather conditions. This is ironically due to the high oil content of residue bitumen which is continually excreted on to the road surface throughout the surface life of the pavement. The slippery conditions become more noticeable over time as the asphalt surface ages and becomes smoother due to the aggregates in the asphalt matrix being polished and smooth rolled by the passing traffic. Motor vehicle tyres have a tendency to pick up this oily surface residue and distribute it from one area to another (binder shift) consequently contributing to slippery road surfaces which is a major road traffic safety hazard .
Motorized vehicles and their drivers Edit
For more information, see article automobile safety
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.
- 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 Edit
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
Non-motorised lobby Edit
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 causalties 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.
Case study: UK pedestrian safety Edit
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 0-85374-494-7).
Motorised lobby Edit
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.
- Automobile safety
- Car accident
- Car safety
- Defensive driving
- Driving behaviour
- Driver distraction
- Driver safety
- Driving under the influence
- Human Factors for Highway Engineers (book)
- Mobile phones and driving safety
- Motor traffic accidents
- Risk compensation
- Traffic psychology
- Transportation safety in the United States
- Transportation accidents
- THINK! Road Safety Campaign website
- AAA Foundation for Traffic Safety
- Accident Analysis and Prevention
- AusRAP - Australian Road Assessment Program
- Associaton for Safe International Road Travel
- BDP - Traffic Psychology
- Centre for Automotive Safety Research
- EuroRAP - European Road Assessment Program
- ERSO - European Road Safety Observatory
- ETSC - European Trasport Safety Council
- Journal of Safety Research
- Human Error in Road Accidents
- Intelligent Communicating Transport Systems
- IRTAD - International Road Traffic and Accident Database
- 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. 1920-1980 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.
Further reading Edit
- Traffic Safety, Leonard Evans, Science Serving Society 2004, ISBN 0975487108
- Death on the Streets: Cars and the mythology of road safety, Robert Davis, Leading Edge 1993, ISBN 0-948135-46-8
- One False Move: a study of children's independent mobility, Mayer Hillman, John Adams, John Whitelegg, Policy Studies Institute 1991, ISBN 0-85374-494-7
- Risk, John Adams, UCL Press 1995, ISBN 1-85728-068-7
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