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Traffic

Traffic code | Driving | Driving on the left or right | Speed limit | Traffic lights | Car safety | Traffic signs

Interstate 80, a freeway in California with many lanes and heavy traffic.

Intersection of 4th and San Fernando in San Jose, California. The intersection has crosswalks, left-turn lanes, and traffic lights.

Traffic refers to the movement of motorized vehicles, unmotorized vehicles and pedestrians on roads. Traffic laws are the laws which govern traffic and regulta vehicles, while rules of the road are both the laws and the informal rules that may have developed over time to facilitate the orderly and timely flow of traffic.

Organized traffic generally has well-established priorities, lanes, right-of-way, and traffic control at intersections.

Organization

In many parts of the world traffic is generally organized, flowing in lanes of travel for a particular direction, with junctions, intersections, interchanges, traffic signals, or signs. Traffic may be separated into classes: vehicular; non-vehicular (e.g. bicycles); and pedestrian. Different classes may share speed limits and easement, or may be segregated. Some countries may have very detailed and complex traffic laws while others rely on drivers' common sense and willingness to cooperate.

Organization typically reduces travel time. Though vehicles wait at some intersections, wait time at others is much shorter. An unexpected occurrence may cause traffic to degenerate into a disorganized mess: road construction, accidents, or debris may all disrupt the flow. On particularly busy freeways, a minor disruption may persist in a phenomenon known as traffic waves. A complete breakdown of organization may result in traffic jams and gridlock. Simulations of organized traffic frequently involve queuing theory, stochastic processes and equations of mathematical physics applied to traffic flow.

Rules of the road

Rules of the road are the general practices and procedures that road users follow, especially motorists and cyclists. They govern interactions with other vehicles and pedestrians. The basic traffic rules are defined by an international treaty under the authority of the United Nations, the 1968 Vienna Convention on Road Traffic. Not all countries are signatory to the convention and, even among signatories, local variations in practice may be found. Driving safely is usually easier if a driver can adapt to both written and unwritten local rules of the road.

These rules should be distinguished from the mechanical procedures required to operate one's vehicle.

Directionality

Main article: Driving on the left or right.

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. 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.

Highway code

In many countries, the rules of the road are codified, setting out the legal requirements and punishments for breaking them.

In the United Kingdom, the rules are set out in the Highway Code, including some obligations, but also a lot of other advice on how to drive sensibly and safely. For this second set of advice, it states: Although failure to comply with the other rules of the Code will not, in itself, cause a person to be prosecuted, The Highway Code may be used in evidence in any court proceedings under Traffic Acts to establish liability. Many of its ex-colonies still retain this notice.

In the United States, traffic laws are regulated by the states and municipalities through their respective traffic code. The federal government's Department of Transportation has some control over road signage and vehicle safety, and limited control over the Interstate highway system (which is actually built and maintained by the states). However, all state vehicle or traffic laws have common elements. These include the mandatory automobile insurance requirement, right-of-way rules, the basic speed rule (go only as fast as is safe under the circumstances up to the maximum posted speed limit), and the requirement to stop after an accident. The most common state-by-state variation is in maximum speed limits; for example, rural states like Montana have speed limits as high as 75 mph (120 km/h), but Oregon has a maximum speed limit of 65 mph (104 km/h) and Hawaii has a maximum of 55 mph. (88 km/h).

Speed limits

Main article: Speed limit

One of the main factors that affect the damage caused by a collision is speed. Therefore, many countries 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.

Priority

As well as the side of the road, priority rules also differ between countries. In the United Kingdom, priority is always indicated by signs or road markings, in that every junction has a concept of a major road and minor road (except those governed by traffic lights). In most of Continental Europe, the default priority is to give way to the right, but this default may be overridden by signs or road markings. In France, until the 1980s, the "priorité à droite" (give way to the right) rule was employed at most roundabouts, in that traffic already on the roundabout had to give way to traffic entering the roundabout. Most French roundabouts now have give-way signs for traffic entering the roundabout, but there remain some notable exceptions that operate on the old rule, such as the Place de l'Étoile around the Arc de Triomphe. Traffic on this particular roundabout is so chaotic that French insurance companies deem any accident on the roundabout to be equal liability. The default give-way-to-the-right rule used in Continental Europe causes problems for many British and Irish drivers who are accustomed to having right of way by default unless they are specifically told to give way.

4-way stop intersections

In the United States and Canada, there are many 4-way intersections with a stop sign at every entrance. In this case, the default rule is:

1. Whichever vehicle stops first has priority.
2. If two vehicles stop at the same time, priority is given to the vehicle on the right.
3. If three vehicles stop at the same time, priority is given to the two vehicles going in the same direction.
4. If four vehicles stop, drivers usually use gestures and other communication to establish right-of-way. In some areas, the custom is for the north-south or the more-trafficked road to have priority, although this is rare.

Overtaking

Overtaking, or passing refers to a maneuver that is in effect passing vehicles traveling in the same direction. On two-lane roads, when there is a split line or a dashed line on the side of the overtaker, drivers may overtake when it is safe. In multi-lane roads in most jurisdictions, overtaking is permitted in the 'slower' lanes. See lanes below.

Lanes

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

When a street is wide enough to accommodate several vehicles traveling 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.

Designation and overtaking

The usual designation for lanes on divided highways is the fastest lane is the one closest to the center of the road, and the slowest to the edge of the road.

When driving on the left:

• The lane designated for faster traffic is on the right
• The lane designated for slower traffic is on the left
• Most freeway exits are on the left
• Overtaking is permitted to the right, and sometimes to the left.

When driving on the right:

• The lane designated for faster traffic is on the left
• The lane designated for slower traffic is on the right
• Most freeway exits are on the right
• Overtaking is permitted to the left, and sometimes to the right.

In the United States, the inside lane refers to the fastest lane, but in the United Kingdom, it refers to the slowest lane.

Usually, drivers are expected to keep in the slowest lane unless overtaking, though with more traffic, all lanes are often used. Many areas in North America do not have any laws about staying to the slowest lanes unless overtaking. In those areas, unlike many parts of Europe, traffic is allowed to overtake on any side, even in a slower lane. This practice is known as passing on the right in the United States, where it is common, overtaking on the inside, and 'undertaking' in the United Kingdom.

U.S. state-specific practices

In some U.S. states such as Massachusetts, although there are laws requiring all traffic on a public way to use the right-most lane unless overtaking, this rule is often ignored and seldom enforced on multi-lane roadways.

In other states like California, cars may use any lane on multi-lane roadways. Slower drivers are strongly encouraged to stay in the right-most lanes to keep the way clear for faster vehicles and thus speed up traffic. However, faster drivers can merely pass in the slower lanes if they wish. But the California Vehicle Code also requires trucks to stay in the right lane, or in the right two lanes if the roadway has four or more lanes going in their direction. The oldest freeways in California that pre-dated this rule often have ramps on the left, making signs like "TRUCKS OK ON LEFT LANE" or "TRUCKS MAY USE ALL LANES" necessary to override the default rule.

Right of way

Vehicles will often come into conflict with other vehicles because their intended courses of travel intersect, and thus interfere with each other's routes. 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 pedestrians 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 priorité-à-droite, or priority to the right when translated to the English language. 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.

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

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.

Expressways

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.

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 dedicated lanes and protected traffic signals for turning are sometimes provided. 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.

One-way streets

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.

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.

In most areas, an intersection is considered to have a crosswalk, even if not painted, as long as the roads meet about approximately right angles.

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.

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.

See also

• Road safety

External links

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

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