Traffic and Road Safety

For other uses, see Traffic (disambiguation).

In many parts of the world traffic is generally organized, flowing in lanes of travel for a particular direction, with interchanges, traffic signals, or signage at intersections to facilitate the orderly and timely flow of traffic. Traffic can be separated into vehicular, non-vehicular (bicycling), and pedestrian classes of traffic. Vehicles of the same class share their speed limits and share easement with one another. Cooperative signaling arrangements can also be made with representatives of other classes of traffic.

Organized traffic typically reduces travel time. Though vehicles wait at some intersections, wait time at others is much shorter. Organized traffic degenerates to disorganized traffic with an unexpected occurrence, be it road construction, an accident, or obstructions in the road such as an animal, debris, or other objects. On particularly busy freeways, a disruption can persist until traffic thins. William Beaty observed persistent disruptions and named the phenomenon traffic waves.

Simulations of organized traffic frequently involve queuing theory, stochastic processes and equations of mathematical physics applied to traffic flow.

Basic Features

Several countries have very complicated traffic laws and others rely on each driver's common sense. However, some basic features of traffic seem to apply to most cultures. Those features are described below.

Directionality

Traffic going in opposite directions should be separated in such a way that they do not block each other's way. The most basic rule regarding this concept is which side of the road should be used for travel. See "Which side?" below for more information. In more sophisticated systems such as large cities, this concept is further extended: some streets are marked as being "one-way", and on those streets all traffic must flow in only one direction. A driver wishing to reach a destination he already passed must use other streets in order to return. Usage of one-way streets, despite the inconveniences it can bring to individual drivers, can greatly improve traffic flow since they usually allow traffic to move faster and tend to simplify intersections.

Lanes

Interstate 80 is a freeway with many lanes and heavy traffic.

When a street is wide enough to accommodate several vehicles travelling side by side, it is usual for traffic to organize itself into "lanes", that is, parallel corridors of traffic. Some roads have one lane for each direction of travel and other have multiple lanes for each direction. Some countries apply pavement markings to clearly indicate the limits of each lane and the direction of travel that it must be used for. In other countries lanes have no markings at all and drivers follow them mostly by instinct rather than visual stimulus.

On roads that have multiple lanes going in the same direction, drivers may usually shift amongst lanes as they please, but they must do so in a way that does not cause inconvenience to other drivers. Driving cultures vary greatly on the issue of "lane ownership": in some countries, drivers traveling in a lane will be very protective of their right to travel in it while on others drivers will routinely expect other drivers to shift back and forth.

Right of Way ("who goes first")

Vehicles will often come into conflict with other vehicles because their intended courses of travel intersect, that is, they get in each other's way. The general principle that establishes who has the right to go first is called "right of way". It establishes who has the right to use the conflicting part of the road and who has to wait until the other driver does so.

Different countries have different rules that establish who has the right of way, but a common pattern is for one of the roads (usually the smaller road) to have a marking indicating that it should "yield" to drivers on the other road. This can be in the form of a STOP sign, dotted lines painted on the pavement or other devices. Drivers approaching from the road with the STOP sign (or equivalent device) are required to stop before the intersection and only proceed when a breach occurs in the other road's traffic. Some countries also include pedestrian crossings near the STOP signs, and in this case the approaching drivers must also allow the pedestrian to cross the street before advancing.

Another way to resolve the right-of-way conflict is to establish a general rule such as the French prioritè-a-droit (priority to the right). This rule establishes that the right of way belongs to the driver who is coming from the right, and the driver coming from the left should yield to him. This rule is unambiguous, but may lead to some counterintuitive situations, such as in T-intersections, where, strangely enough, traffic going straight through the top segment of the T must yield to entering traffic that comes from the vertical leg of the T.

Police regulate the traffic ...

... whenever traffic lights fail on busy streets in Vienna, Austria.

In most modern cities the traffic signal is used to establish the right of way on the busy roads. Its primary idea is to give each road a slice of time in which its traffic may use the intersection in an organized way. The intervals of time assigned for each road may be adjusted to take into account factors such as difference in volume of traffic.

Turning

Vehicles will often want to cease to travel in a straight line and turn onto another road. The vehicle's directional signals (blinkers) are often used as a way to announce one's the intention to turn, thus alerting other drivers. The actual usage of blinkers vary greatly amongst countries. Turning traffic must usually yield the right of way to oncoming traffic - on right-driving countries, vehicles must yield when performing a left turn; on left-driving countries vehicles must yield when performing a right turn. This will usually mean that turning traffic will have to stop in order to wait for a breach to turn, and this might cause inconvenience for vehicles that follow them but do not want to turn. This is why sometimes "protected lanes" for turning are provided, that is, a special lane where vehicles can wait without standing in the way of traffic. On busier intersections where a protected lane would be ineffective or cannot be built, turning may be entirely prohibited, and drivers will be required to "drive around the block" in order to accomplish the turn.

On roads with multiple lanes, turning traffic is generally expected to move to the lane closest to the direction they wish to turn. For example, traffic intending to turn right will usually move to the rightmost lane before the intersection. Likewise, left-turning traffic will move to the leftmost lane. Exceptions to this rule may exist where for example the traffic authority decides that the two rightmost lanes will be for turning right, in which case drivers may take whichever of them to turn. On certain parts of the world traffic will adapt to informal patterns that rise naturally rather than by force of authority: for example, in Brazil and elsewhere it is common for drivers to observe (and trust) the turn signals used by other drivers in order to make turns from other lanes. For example if several vehicles on the right lane are all turning right, a vehicle may come from the next-to-right lane and turn right as well, doing so in parallel with the other right-turning vehicles.

Pedestrian Crossings

A picture of Avenida Faria Lima in São Paulo, Brazil, showing a semaphore-controlled pedestrian crossing, and several red lights on several intersections ahead.

Pedestrians must often cross from one side of a road to the other, and in doing so may come into the way of vehicles traveling on the road. In many places pedestrians are entirely left to look after themselves, that is, they must observe the road and cross when they can see that no traffic will threaten them. Busier cities usually paint "pedestrian crossings", which are strips of the road where pedestrians are expected to cross.

The actual appearance of pedestrian crossings varies greatly, but the two most common appearances are: (1) a series of parallel white stripes or (2) two long horizontal white lines. The former is usually preferred, as it stands out more conspicuously against the dark pavement.

Some pedestrian crossings also accompany a traffic signal which will make vehicles stop at regular intervals so the pedestrians can cross. Some countries have "intelligent" pedestrian signals, where the pedestrian must push a button in order to assert his intention to cross. The traffic signal will use that information to schedule itself, that is, when no pedestrians are present the signal will never pointlessly cause vehicle traffic to stop.

Pedestrian crossings without traffic signals are also common. In this case, the traffic law usually states that the pedestrian has the right of way when crossing, and that vehicles must stop when a pedestrian uses the crossing. Countries and driving cultures vary greatly as to the extent to which this is respected.

Traffic Pre-emption

In some areas, emergency responders are provided with specialized equipment which allows emergency response vehicles, particularly fire fighting apparatus, to have high-priority travel, by changing the lights in their corridor to green and intersecting streets along the corridor to red. The technology behind these methods have evolved, from panels at the fire department that could trigger and control green lights for certain major corridors, to optical systems, which the individual fire apparatus can be equipped with to communicate directly with receivers on the signal head.

Intelligent Transportation Systems

Intelligent Transportation Systems (ITS) is a system of hardware, software and operators that allow better monitoring and control of traffic in order to optimize traffic flow. As the number of vehicle lane miles traveled per year continues to increase dramatically, and as the number of vehicle lane miles constructed per year has not been keeping pace, this has led to ever-increasing traffic congestion. As a cost-effective solution toward optimizing traffic, ITS presents a number of technologies to reduce congestion by monitoring traffic flows through the use of sensors and live cameras, and in turn rerouting traffic as needed through the use of variable message boards (VMS), highway advisory radio (HAR) and other systems. Additionally, the roadway network has been increasingly fitted with additional communications and control infrastructure to allow traffic operations personnel to monitor weather conditions, for dispatching maintenance crews to perform snow or ice removal, as well as intelligent systems such as automated bridge de-icing systems which help to prevent accidents.

Speed

One of the main factors that affect the damage caused by a collision is speed. Therefore, most civilized parts of the world impose speed limits on their roads. Drivers are not supposed to drive at speeds which are higher than the posted limit.

To enforce the speed limit, two approaches are generally employed. In the USA it is common for the police to patrol the streets and use special equipment to measure the speed of vehicles, and "pull over" any vehicle found to be in violation of the speed limit. In Brazil and some European countries, there are computerized speed-measuring devices spread throughout the city, which will automatically detect speeding drivers and take a photograph of the license plate, which is later used for applying and mailing the ticket.

Another interesting mechanism that was developed in Germany is the Grüne Welle, or green wave, which is an indicator that shows the optimal speed to travel for the synchronized green lights along that corridor. This encourages drivers to travel at the posted limit in order to minimize stopping.

Expressways

Prestes Maia Expressway, in São Paulo, Brazil, near rush hour, already showing some considerable traffic density.

In large cities, moving from one part of the city to another by means of ordinary streets and avenues can be time-consuming since traffic usually moves at slow speeds and there are many intersections, stop signs, parked cars, pedestrian crossings, bicycle traffic and other obstacles. Therefore, it has become common practice for larger cities to build expressways, which are large and wide avenues that run for long distances and have no intersections or semaphores. Vehicles wishing to travel over great distances within the city will usually take the expressways in order to save on travel time. When another road must cross an expressway, a bridge will be built if the expressway is a ground-level road, or it will pass under the expressway if it is elevated.

Expressways usually have controlled entry and exit, that is, entering and leaving the expressway may only be done at specific points called entries and exits. Vehicles entering the expressway must yield the right of way to the vehicles already traveling on it.

Unorganized traffic

Unorganized traffic occurs in the absence of lanes and signals. Roads do not have lanes, though drivers tend to keep to the appropriate side if the road is wide enough. Drivers frequently overtake other drivers, and obstructions are not uncommon.

Intersections have no signals or signage, and a particular road at a busy intersection may be dominant (that is, its traffic flows) until a break in traffic, at which time the dominance shifts to the other road where vehicles are queued. At the intersection of two perpendicular roads, a traffic jam results if four vehicles face each other side-on.

Which side?

According to Brian Lucas, about 34% of the world by country population drives on the left, and 66% keeps right. By roadway miles, about 72% drive on the right.

See also

• Rules of the road
• Traffic psychology
• Traffic congestion
• Transport

External links

Wikimedia Commons has media related to:

Traffic

• Institute of Transportation Engineers (ITE)
• Traffic Waves
• Better Environmentally Sound Transportation
• Welcome to GreenerCars.com: The online home of ACEEE's Green Book
• Traffic calming in a community
• Traffic.com

The field of road safety is concerned with reducing the numbers or the consequences of vehicle crashes, by developing and implementing management systems ideally based in a multidisciplinary and holistic approach, with interrelated activities in a number of fields. This has not always been the case, some historical road safety initiatives were based on overly simplistic models of driver behaviour.

History

Crashes seem as old as automobile vehicles themselves. Nicolas-Joseph Cugnot crashed his steam-powered "Fardier" against a wall in 1770. The first recorded automobile fatality was Bridget Driscoll in 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, 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 motorized user being, perhaps, impaired.

Defining the problem

The standard measures used in assessing road safety interventions are fatalities and Killed or Seriously Injured (KSI) rates, usually per billion passenger kilometres.

Speed is a key goal of modern road design, but impact speed determines 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).

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 δv of roughly double 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

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.

source: International Road Traffic and Accident Database (IRTAD); all countries listed with overall fatality rates.

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, motor 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

Interventions take many forms.

Road design

On neighborhood roads with young pedestrians, traffic calming can be a tool for road safety. Recent experiments have gone much further, removing all distinction between carriageway (roadway) and footway (sidewalk), an initiative popularly known as naked streets. Both approaches can be shown to be effective.

At the other end of the spectrum, motorways (which may be called limited access highways, Autobahns, Interstates or other national names) achieve safety by:

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

Unsurprisingly, then, motorways are often the safest roads per mile travelled (see table below) while at the same time delivering higher speeds and, generally, smoother flow of traffic.

source: International Road Traffic and Accident Database (IRTAD) -- for all countries reporting fatality rates overall and on motorways, and reporting fatality counts overall and on motorways.

Given the lack of conflicting traffic and relatively homogeneous traffic speeds, motorway casualty rates may approximate pure driver-related error rates.

However, motorways are far more expensive to build than ordinary roads and are only really practical for arterial routes. Motorways around the world are subject to a 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; smoother traffic flow reduces collisions and improves fuel economy, although this also reduces with increasing speed.

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.

Road hazards and intersections 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 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.

Drivers and vehicles

It has been said, with good reason, that the problem of road safety is largely a problem of the private car. Safety interventions focusing on the driver and vehicle 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.
• 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 thing some countries require periodic retests and others will require drivers convicted of offences to undergo extensive training and retests before being allowed back on the roads).
• Restrictions on driving while drunk or impaired by drugs.
• 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.

Many of these interventions have been opposed by the car manufacturers (see Unsafe at Any Speed) or by drivers themselves. Britain's largest motoring organsiation, the AA, was originally set up to warn members of the presence of speed traps, likened in one parliamentary debate to "a society of burglars employing scouts".

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' 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 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 that the problem of road safety is largely being stated in the wrong terms. 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).

The basis of these arguments can be understood by reference to some of the road safety interventions targeted at improving the safety of non-motorised road 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.

Pedestrians in particular are often reluctant ot use segregated faciltiies 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.

Other interventions are more obviously to the benefit of vulnerable road users, but often opposed by the motor lobby:

• traffic calming and speed humps
• naked streets and home zones, giving ownership of the common space back to local residents
• reduced urban speed limits

Case study: UK pedestrian safety

The "road safety" establishment is proud of the fact that the UK has 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

So there is some evidence at least to support the contention that Britain's roads are not in fact particularly safe at all, it is just that the vulnerable are too intimidated to use them.

See also

• List of famous people who died in road accidents
• Human Factors for Highway Engineers (book)
• Air safety
• Automated Highway System
• Car safety
• Rubbernecking
• Defensive driving
• Management systems for road safety
• Risk compensation
• Stop sign
• Traffic psychology

External links

• AAA Foundation for Traffic Safety
• Accident Analysis and Prevention
• Journal of Safety Research
• Human Error in Road Accidents
• Official UK government website on road safety
• English version of the International Road Traffic and Accident Database (IRTAD), 2003 Risk Values, courtesy of the Bundesanstalt für Straßenwesen, that is, the (German) Federal Highway Research Institute

Further reading

• Death on the Streets: Cars and the mythology of road safety, Robert Davis, Leading Edge 1993, ISBN: 0948135468
• One False Move: a study of children's independent mobility, Mayer Hillman, John Adams, John Whitelegg, Policy Studies Institute 1991, ISBN: 0853744947
• Risk, John Adams, UCL Press 1995, ISBN: 1857280687