are on the left and
miles per hour on the right. s around the world: Most countries use the
metric speed unit of
kilometres per hour, while others, including the
United States and
United Kingdom and
Belize, use speed limits given in
miles per hour.
Vienna Convention on Road Traffic In countries bound by the
Vienna Conventions on Road Traffic (1968 & 1977), Article 13 defines a basic rule for speed and distance between vehicles:
Reasonable speed Most legal systems expect drivers to drive at a safe speed for the conditions at hand, regardless of posted limits. In the United Kingdom, and elsewhere in
common law, this is known as the
reasonable man requirement. The German Highway Code (
Straßenverkehrs-Ordnung) section on speed begins with the statement (translated to English): Any person driving a vehicle may only drive so fast that the car is under control. Speeds must be adapted to the road, traffic, visibility and weather conditions as well as the personal skills and characteristics of the vehicle and load. In France, the law clarifies that even if the speed is limited by law and by local authority, the driver assumes the responsibility to control a vehicle's speed, and to reduce that speed in various circumstances (such as when overtaking a pedestrian or bicycle, individually or in a group; when overtaking a stopped convoy; when passing a transportation vehicle loading or unloading people or children; when the road does not appear clear, or risky; when visibility is low, etc.). If drivers do not control their speed, or do not reduce it in such cases, they can be penalized. Other qualifying conditions include driving through fog, heavy rain, ice, snow, gravel, or when drivers encounter sharp corners, a blinding glare, darkness, crossing traffic, or when there is an obstructed view of orthogonal traffic—such as by road curvature, parked cars, vegetation, or snow banks—thus limiting the
Assured Clear Distance Ahead (ACDA). In the United States, this requirement is referred to as the
basic rule, as outlined by US federal government law (49 CFR 392.14), which applies in all states as permitted under the
commerce clause and
Due Process Clause. The basic speed law is almost always supplemented by specific maximum or minimum limits but applies regardless. In California, for instance, Vehicle Code section 22350 states that "No person shall drive a vehicle upon a highway at speed greater than is reasonable... and in no event at a speed which endangers the safety of persons or property". The reasonable speed may be different than the posted speed limit.
Basic rule speed laws are statutory reinforcements of the centuries-old
common law negligence doctrine as specifically applied to vehicular speed. Citations for violations of the basic speed law without a crash have sometimes been ruled unfairly vague or arbitrary, hence a violation of the
Due Process Clause, at least in the State of Montana. Even within states, differing jurisdictions (counties and cities) choose to prosecute similar cases with differing approaches.
Excessive speed Consequential results of basic law violations are often categorized as
excessive speed crashes; for example, the leading cause of
crashes on German autobahns in 2012 fell into that category: 6,587 so-called "speed related" crashes claimed the lives of 179 people, which represented almost half (46.3%) of 387 autobahn fatalities in 2012. However, "excessive speed" does not necessarily mean the speed limit was exceeded, rather that police determined at least one party traveled too fast for existing conditions. Examples of conditions where drivers may find themselves driving too fast include wet roadways (due to rain, snow, or ice), reduced visibility (due to fog or "white out" snow), uneven roads, construction zones, curves, intersections, gravel roads, and heavy traffic. Per distance traveled, consequences of inappropriate speed are more frequent on lower speed, lower quality roads; in the United States, for example, the "speeding fatality rate for local roads is three times that for Interstates". For speed management, a distinction can exist between
excess speed, which consists of driving in excess of the speed limit, and
inappropriate speed, which consists of going too fast for the conditions.
Maximum speed limits Most countries have a legally assigned numerical maximum speed limit which applies on all roads when no other speed limit indications are present; lower speed limits are often shown on a sign at the start of the restricted section, although the presence of streetlights or the physical arrangement of the road may sometimes also be used instead. A posted speed limit may only apply to that road or to all roads beyond the sign that defines them depending on local laws. The speed limit is commonly set at or below the
85th percentile speed (the operating speed which no more than 15% of traffic exceeds), and in the US is frequently set below that speed. Thus, if the 85th percentile
operating speed as measured by a "Traffic and Engineering Survey" exceeds the
design speed, legal protection is given to motorists traveling at such speeds (design speed is "based on conservative assumptions about the driver, the vehicle, and roadway characteristics"). The theory behind the 85th percentile rules is that, as a policy, most citizens should be deemed reasonable and prudent, and limits must be practical to enforce. However, there are some circumstances where motorists do not tend to process all the risks involved, and as a mass, choose a poor 85th percentile speed. This rule, in practice, is a process for "voting the speed limit" by driving, in contrast to delegating the speed limit to an engineering expert. The maximum speed permitted by statute, as posted, is normally based on ideal driving conditions and the basic speed rule always applies. Violation of the statute generally raises a
rebuttable presumption of negligence. On international European roads, speed should be taken into account during the design stage.
Minimum speed limits Some roads also have minimum speed limits, usually where slow speeds can impede traffic flow or be dangerous. The use of minimum speed limits is not as common as maximum speed limits, since the risks of speed are less common at lower speeds.
Middle speed limits Traffic rules limiting only middle speeds are rare. One such example exists on the
ice roads in
Estonia, where it is advised to avoid driving at the speed of as the vehicle may create resonance that may in turn induce the breaking of ice. This means that two sets of speeds are allowed: under and between .
Variable speed limits In Germany, the first known experiments with variable speed limit signs took place in 1965 on a stretch of German motorway, the
A8 between
Munich and the border city of
Salzburg, Austria. Mechanically variable message signs could display speeds of 60, 80 and 100 km/h, as well as text indicating a "danger zone" or "accident". Personnel monitored traffic using video technology and manually controlled the signage. Beginning in the 1970s, additional advanced traffic control systems were put into service. Modern motorway control systems can work without human intervention using various types of sensors to measure traffic flow and weather conditions. In 2009, of German motorways were equipped with such systems. In the United States, heavily traveled portions of the
New Jersey Turnpike began using variable speed limit signs in combination with
variable message signs in the late 1960s. Officials can adjust the speed limit according to weather, traffic conditions, and construction. More typically, variable speed limits are used on remote stretches of highway in the United States in areas with extreme changes in driving conditions. For example, variable limits were introduced in October 2010 on a stretch of
Interstate 80 in
Wyoming, replacing the winter season speed reduction from that had been in place since 2008. This Variable Speed Limit system has been proven effective in terms of reducing crash frequency and road closures. Similarly,
Interstate 90 at
Snoqualmie Pass and other mountain passes in Washington State have variable speed limits as to slow traffic in severe winter weather. As a response to fog-induced chain-reaction collisions involving 99 vehicles in 1990, a variable speed limit system covering of
Interstate 75 in
Tennessee was implemented in fog-prone areas around the
Hiwassee River. The
Georgia Department of Transportation installed variable speed limits on part of
Interstate 285 around
Atlanta in 2014. These speeds can be as low as but are generally set to . In 2016, the
Oregon Department of Transportation installed a variable speed zone on a stretch of
Interstate 84 between
Baker City and Ladd Canyon. The new electronic signs collect data regarding temperature, skid resistance, and average motorist speed to determine the most effective speed limit for the area before presenting the limit on the sign. This speed zone was scheduled to be activated November 2016.
Ohio established variable speed limits on three highways in 2017, then in 2019 granted the authority to the
Ohio Department of Transportation to establish variable limits on any of its highways. In the United Kingdom, a variable speed limit was introduced on part of the
M25 motorway in 1995, on the busiest section from junction 10 to 16. Initial results suggested savings in journey times, smoother-flowing traffic, and a decrease in the number of crashes; the scheme was made permanent in 1997. However, a 2004 National Audit Organisation report noted that the business case was unproved; conditions at the site of the Variable Speed Limits trial were not stable before or during the trial, and the study was deemed neither properly controlled nor reliable. Since December 2008 the
upgraded section of the M1 between the M25 and
Luton has had the capability for variable speed limits. In January 2010 temporary variable speed cameras on the M1 between J25 and J28 were made permanent. New Zealand introduced variable speed limits in February 2001. The first installation was on the
Ngauranga Gorge section of the dual carriageway on
State Highway 1, characterized by steep terrain, numerous bends, high traffic volumes, and a higher than average accident rate. The speed limit is normally .
Austria undertook a short-term experiment in 2006, with a variable limit configuration that could increase statutory limits under the most favorable conditions, as well as reduce them. In June 2006, a stretch of motorway was configured with variable speed limits that could increase the general Austrian motorway limit of . Then Austrian Transport Minister
Hubert Gorbach called the experiment "a milestone in European transport policy-despite all predictions to the contrary"; however, the experiment was discontinued.
Roads without speed limits Just over half of the
German autobahns have only an advisory speed limit (a
Richtgeschwindigkeit), 15% have temporary speed limits due to weather or traffic conditions, and 33% have permanent speed limits, according to 2008 estimates. The advisory speed limit applies to any road in Germany outside of towns which is either a dual carriageway or features at least two lanes per direction, regardless of its classification (e.g. Autobahn, Federal Highway, State Road, etc.), unless there is a speed limit posted, although it is less common for non-autobahn roads to be unrestricted. All other roads in Germany outside of towns, regardless of classification, do have a general speed limit of , which is usually reduced to at Allée-streets (roads bordered by trees or bushes on one or both sites). Travel speeds are not regularly monitored in Germany; however, a 2008 report noted that on the autobahn in
Niemegk (between Leipzig and Berlin) "significantly more than 60% of road users exceed [and] more than 30% of motorists exceed ". Measurements from the state of
Brandenburg in 2006 showed average speeds of on a 6-lane section of autobahn in free-flowing conditions. Prior to
German reunification in 1990, accident reduction programs in
eastern German states were primarily focused on restrictive traffic regulation. Within two years of reunification, the availability of high-powered vehicles and a 54% increase in motorized traffic led to a doubling of annual traffic deaths, despite "interim arrangements [which] involved the continuation of the speed limit of on autobahns and of outside cities". An extensive program of the four
Es (enforcement, education, engineering, and
emergency response) brought the number of traffic deaths back to pre-unification levels after a decade of effort, while traffic regulations were conformed to western standards (e.g., freeway advisory limit, on other rural roads).
Many rural roads on the Isle of Man have no speed limits;
Measured travel speeds on the island are relatively low. The Indian states of
Andhra Pradesh,
Maharashtra, and
Telangana also do not have speed limits by default.
Roads formerly without speed limits Many roads without a maximum limit became permanently limited following the
1973 oil crisis. For example,
Switzerland and
Austria had no maximum restriction prior to 1973 on motorways and rural roads, but imposed a temporary maximum limit in response to higher fuel prices; the limit on motorways was increased to later in 1974.
Montana and
Nevada were the last remaining U.S. states relying exclusively on the basic rule, without a specific, numeric rural speed limit before the
National Maximum Speed Law of 1974. After the repeal of federal speed mandates in December 1995, Montana was the only state to revert to the basic rule for daylight rural speed regulation. The
Montana Supreme Court ruled that the basic rule was too vague to allow citation, prosecution, and conviction of a driver; concluding enforcement was a violation of the
due process requirement of the
Montana Constitution. In response, Montana's legislature imposed a limit on rural freeways in 1999. Australia's
Northern Territory had no rural speed limit until 2007, and again from 2014 to 2016. Sections of the Stuart Highway had no limits as part of an
open speed limit trial. ==Method==