Traffic is the movement of motorized vehicles, unmotorized vehicles and
pedestrians on roads. Traffic laws are the
laws which govern traffic and regulate 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
Traffic is formally organized in many jurisdictions, with marked lanes, junctions, intersections, interchanges, traffic signals, or signs. Traffic is often classified by type: heavy motor vehicle (e.g. car, truck); other vehicle (e.g. moped, bicycle); and pedestrian. Different
classes may share speed limits and easement, or may be segregated. Some jurisdictions may
have very detailed and complex rules of the road while others rely more on drivers' common sense and willingness to
cooperate.
Organization typically produces a better combination of travel safety and efficiency. Events which disrupt the flow and may
cause traffic to degenerate into a disorganized mess include: road construction, collisions, and debris in the roadway. 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
Traffic control in
Rome,
Italy. This traffic control podium can
retract back to road level when not in use.
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.
As a general rule, a driver is expected to avoid hitting other vehicles, pedestrians, etc. regardless of whether or not the
applicable rules of the road allow them to be where they happen to be.
In addition to the rules applicable by default there are traffic signs, including
traffic lights, and instructions may be given by a police officer, either routinely on a
busy crossing instead of traffic lights, or as road traffic control around a
construction zone, accident or other road disruption.
These rules should be distinguished from the mechanical procedures required to operate one's vehicle. See driving.
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, some states like Texas
have speed limits as high as 80 mph (130 km/h), with 75 mph (120 km/h) being more common, but Oregon has a maximum speed limit of 65 mph (104 km/h) and Hawaii has a
maximum of 60 mph. (97 km/h).
Speed limits
- Main article: Speed limit
The higher the speed of a vehicle, the more difficult collision avoidance becomes and the greater the damage if a collision
does occur. Therefore, many countries of the world limit the maximum speed allowed on their
roads. Vehicles are not supposed to be driven at speeds which are higher than the posted maximum.
To enforce speed limits, two approaches are generally employed. In the USA, it is
common for the police to patrol the streets and use special equipment (Typically a RADAR Gun) 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 (or number 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. See related traffic wave.
Priority (right of way)
A diagram of movement within a
roundabout in a country where traffic drives on the left. A
roundabout is a type of
road junction, or
traffic
calming device, at which
traffic streams circularly around a central island after first
yielding to the circulating traffic. Unlike with traffic circles, vehicles on a roundabout have priority over the entering
vehicle,
parking is not allowed and
pedestrians are
usually prohibited from the central island.
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", or "priority". 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.
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, priority was initially
according to the social rank of each traveler, but early in the life of the automobile this
rule was deemed impractical and replaced with the "priorité à droite" (priority to the right) rule, which was employed until the
1980s. At a roundabout, "priorité à droite" works this way: traffic already on the roundabout gives 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.[citation needed]
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.
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" or "give way" 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 gap 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.
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.
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:
- Whichever vehicle stops first has priority.
- If two vehicles stop at the same time, priority is given to the vehicle on the right.
- If three vehicles stop at the same time, priority is given to the two vehicles going in opposite directions.
- 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 manoeuvre 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.
In the United Kingdom and United States,
notably on extra-urban roads, a solid white or yellow line closer to the driver is used to indicate that no overtaking is allowed
in that lane. A double white or yellow line means that neither side may overtake.
Lanes
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 others 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 in 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 in Massachusetts and
New York, 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 California, cars may use any lane on multi-lane roadways. Drivers
moving slower than the general flow of traffic are required to stay in the right-most lanes (by California Vehicle Code (CVC) 21654) to keep the way clear for faster vehicles and thus speed up traffic. However,
faster drivers may legally pass in the slower lanes if conditions allow (by CVC 21754). But the CVC 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, and some freeway
interchanges, 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. Lane splitting, or riding motorcycles in the space between
cars in traffic, is permitted as long as it is done in a safe and prudent manner.[1]
Expressways and freeways
- Main articles: Expressway and Freeway
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 is often slowed by at-grade junctions, tight turns, narrow marked lanes
and lack of a minimum speed limit. Therefore, it has become common practice for larger
cities to build expressways or freeways, which are large and
wide roadways with limited access, that typically run for long distances without at-grade junctions.
The words expressway and freeway have varying meanings in different jurisdictions and in popular use in
different places; however, there are two different types of roads used to provide high-speed access across urban areas:
- The freeway (in USA usage) or motorway in UK usage, is a
divided multi-lane highway with fully-controlled access and grade-separated intersections (no stops). Some freeways are called
expressways, super-highways, or turnpikes, depending on local usage. Access to freeways is fully controlled;
entering and leaving the freeway is permitted only at grade-separated interchanges.
- The expressway (when the name does not refer to a freeway or motorway) is usually a broad
multi-lane avenue, frequently divided, with some grade-level intersections (although usually only where other expressways or
arterial roads cross).
Motor vehicle drivers wishing to travel over great
distances within the city will usually take the freeways or expressways in order to minimize travel time. When a crossing road is
at the same grade as the freeway, a bridge
(or, less often, an underpass) will be built for the crossing road. If the freeway is elevated,
the crossing road will pass underneath it.
Minimum speed signs are sometimes posted (although increasingly rare) and usually indicate that any vehicle traveling slower
than 40 mph (~65 km/h) should indicate a slower speed of travel to other motorvehicles by engaging the vehicle's four-way
flashing lights. Alternative slower-than-posted speeds may be in effect, based on the posted speed limit of the
highway/freeway.
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 intention to turn, thus alerting other drivers. The actual usage of blinkers
varies greatly amongst countries, although its purpose should be the same in all countries: to indicate a driver's intention to
depart from the current (and natural) flow of traffic well before the departure is executed (typically 3 seconds as a
guideline).
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.
Many cities employ this tactic quite often; in San Francisco, due to its
common practice, making three right turns is known colloquially as a "San Francisco left turn". Likewise, as many intersections
in Taipei City are too busy to allow direct left turns, signs often direct drivers to drive
around the block to turn.
Turning rules are by no means universal. In New Zealand for example, left turning traffic must give way to opposing 'right
turning' traffic i.e. traffic turning into your path (unless there are multiple lanes to turn into). That rule often confuses
tourists (and most local drivers as well).
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 two rightmost lanes will be 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, but pedestrians on the sidewalks are generally
not limited to one-way movement. 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 provide
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 laws 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.
Some jurisdictions forbid crossing or using the road anywhere other than at crossings, termed jaywalking. In other areas, pedestrians may have the right to cross where they choose, and have right of
way over vehicular traffic while crossing.
In most areas, an intersection is considered to have a crosswalk, even if not painted, as long as the roads meet at
approximate right angles. One example of a location where this rule is not in effect is the United Kingdom.
Level crossings
-
An example of a typical rail crossing in the United States.
A level crossing is an at-grade intersection of a railway by a road. Because of safety issues, they are often equipped with
closable gates, crossing bells and warning signs.
Rush hour
Prestes Maia Expressway, in
São Paulo,
Brazil, near rush hour,
already showing some considerable traffic density.
During business days in most major cities, traffic congestion reaches great
intensity at predictable times of the day due to the large number of vehicles using the road at the same time. This phenomenon is
called rush hour, although the period of high traffic intensity may exceed one hour.
Rush hour policies
Some cities adopt policies to reduce rush-hour traffic and pollution and encourage the use of public transportation. For
example, in São Paulo, Brazil each vehicle has a specific day
of the week in which it is forbidden from traveling the roads during rush hour. The day for each vehicle is taken from the
license plate number, and this rule is enforced by traffic police and also by hundreds of strategically positioned traffic
cameras backed by computerized image-recognition systems that issue tickets to offending drivers.
In the United States and Canada, several expressways have a special lane (called an "HOV Lane" - High Occupancy Vehicle Lane) that can only be used by cars carrying two (some
locations-three) or more people, and several cities offer a public telephone service where citizens can arrange rides with others
depending on where they live and work. The purpose of these policies is to reduce the number of vehicles on the roads and thus
reduce rush-hour traffic intensity.
Uncontrolled traffic
Uncontrolled traffic occurs in the absence of lane markings and traffic control signals. On roads without marked lanes, drivers tend to keep
to the appropriate side if the road is wide enough. Drivers frequently overtake others. 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 having the lights along their route change to green. The technology behind these methods have evolved,
from panels at the fire department (which 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). In other
areas, public transport buses have special equipment to get green lights.
During emergencies where evacuation of a heavily populated area is required, local authorities may institute contraflow lane reversal, in which all lanes of a road lead away from a danger zone regardless
of their original flow. Aside from emergencies, contraflow may also be used to ease traffic congestion during rush hour or at the
end of a sports event (where a large number of cars are leaving the venue at the same time). For example, the six lanes of the
Lincoln Tunnel can be changed from three in-bound and three out-bound to a two/four
configuration depending on traffic volume.
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 or analysing cellular phone data travelling in cars (Floating Cellular Data, FCD) and in turn rerouting traffic as needed through the use of variable
message boards (VMS), highway advisory radio (HAR), on board or off board navigation devices 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
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References
External links
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