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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.


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.


Interstate 80 is a freeway with many lanes and heavy traffic.
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": on some countries, drivers traveling on a lane will be very protective of their right to travel on 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.
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.


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.
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. On 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.


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.


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

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