Speed limits in Australia

Speed limits in Australia range from 5 km/h shared zones to 130 km/h. Speed limit signage is in km/h since metrication on 1 July 1974. All speed limits (with the sole exception of the South Australian school and roadworks zones which are signposted at 25 km/h) are multiples of 10 km/h – the last digit in all speed signs is zero. Speed limits are set by state and territory legislation albeit with co-ordination and discussion between governments.

Common limits
Australian states and territories use two "default" speed limits. These apply automatically in the absence of 'posted' speed restriction signage. The two default speed limits are:
 * within built-up areas, 50 km/h, except for the Northern Territory which remains at 60 km/h
 * outside built-up areas, 100 km/h;

Common speed zones below the default built up area 50 km/h limit are:
 * Shared zones (signposted areas where pedestrians and motorised traffic share the same space) are 10 km/h.
 * School zones are variable speed zones, with a 40 km/h limit applying during gazetted school terms (which may include pupil-free days) and at specific times of the day when children are expected to be present. In South Australia, the limit is 25 km/h. A minority of school zones have flashing lights to indicate when the lower speed limit applies. In Western Australia, every school has a flashing speed zone sign, most of which are solar powered.
 * 40 km/h zones. A number of local governments have implemented lower speed limits, typically 40 km/h, in certain areas, such as shopping precincts, whole suburbs such as Balmain and Rozelle in Sydney, or areas with high pedestrian activity.

Common speed zones above the default limits are:
 * Many sub-arterial roads are zoned 60 km/h.
 * Major connector roads and smaller highways are zoned 60 km/h, 70 km/h, 80 km/h or 90 km/h.
 * Some highways and freeways are zoned 110 km/h.
 * Most of the Stuart, Arnhem, Barkly and Victoria highways in the Northern Territory are zoned 130 km/h.

The "END" speed limit sign is increasingly used throughout Australia to signal the end of a posted speed restriction, or built-up area "default" speed-limit leading to the jurisdiction's "rural" default speed limit. It contains the word "END" and a number in a black circle beneath this, representing the ceasing speed-limit. It is typically used where, according to AS1742.4 the road beyond has certain hazards such as hidden driveways, poor camber, soft edges and other hazards where the road authority feels a posted speed limit sign might be too dangerous or otherwise unwarranted. It is intended therefore to invoke particular caution. This sign is used as a direct replacement for the slash-through speed derestriction signs common in Europe and elsewhere.

Speed limits are enforced in all areas of the country. Tolerance is about 6 km/h in urban areas and 9 km/h on highways depending on regulations of respective state such that driver driving on or below the tolerance speed will not receive a ticket. This is the case in South Australia and other states have similar tolerances. One exception is Victoria where they will deduct 2–4 km/h in from the speed reading such that reasonable doubt is credited to the driver. especially in light of the fact that earlier Australian Design Rules specified that vehicle speedometers may have up to 10% leeway in accuracy. This was updated in 2006 to require that the "speed indicated shall not be less than the true speed of the vehicle." Detection measures used are radar, LIDAR, fixed and mobile speed cameras (using various detection technologies), Vascar, pacing and aircraft.

Default speed limits by state and territory
Despite introduction of model national road rules by the states in 1999, Western Australia and the Northern Territory retain different default speed limits. The table below indicates the default speed limits along with typical school zone limits and the highest zone in each locality.

In the external territories, and in some special cases (such as Lord Howe Island, NSW), the speed limits may differ significantly from those found across the rest of the nation.

Limits for Learner, Provisional and Probationary licence holders
The table below indicates the different speed limits that apply for each state's licence holders.

The limits apply when the roads stated speed is above the licence's speed limit, e.g.: a person with a NSW P1 licence can drive at a max speed of 90 km/h on 100/110 km/h signed roads in any state. Conversely, a person with a Queensland provisional licence is not speed limited and can drive at the road's stated speed, even while in NSW where local provisional drivers are limited to 90 km/h.

Limits for trucks and buses
New South Wales has a limit of 100 km/h for heavy (more than 4.5 tonne) vehicles which is not a feature of the Australian Road Rules. Additionally particular heavy vehicles (some buses and vehicles more than 13.9 tonne) are required to have speed monitoring devices fitted.

Historical limits
Historically, Australia operated a simple speed limit system of urban and rural default limits, denoted in miles per hour. As part of metrication in 1974, speed limits and speed advisories were converted into kilometres per hour, rounded to the nearest 10 km/h, leading to small discrepancies in speed limits. Also, the signage was changed from the design where the words "SPEED LIMIT" appear above the numeric limit (as specified in the current US MUTCD) to the design where the numeric limit is inscribed on a red circle (which is defined by the Vienna Convention and thus it became an international sign for speed limit).

Urban limits
The urban default, which prior to the 1930s was 30 mph, applied to any "built up area", usually defined by the presence of street lighting. Over the next 30 years, each of the states and territories progressively increased the limit to 35 mph, with New South Wales being the last to change in May 1964. South Australia adopted 35 mph on 30 November 1950, along with the "new short-right hand turn" in place of a hook turn.

Metrication led to the default urban limits of 35 mph being converted to 60 km/h, an increase of 3.7 km/h.

In the late 1990s and early 2000s, the 60 km/h urban default limit was progressively lowered to 50 km/h nationally for reasons of road, and especially pedestrian, safety. However, many existing roads, especially subarterial roads in urban areas, have had 60 km/h limits posted on them. Queensland's Manual of Uniform Traffic Control Devices (Speed Controls) states that 60 km/h is the general minimum speed limit for traffic-carrying roads. The Northern Territory has retained the 60 km/h limit; however, 50 km/h is also a common speed limit (particularly in residential areas).

Rural limits
Outside of built up areas, a prima facie speed limit applied. In New South Wales and Victoria, speed limit was 50 miles per hour (80 km/h in New South Wales  after metrication). In the 1970s however, most state speed limits were gradually replaced by absolute limits. An absolute speed limit of 70 mph was introduced to Victoria in 1971, as a trial. This was subsequently reduced to 60 mph in late 1973. South Australia introduced an absolute speed limit of 65 mph in 1974.

With metrication in 1974, the rural defaults of 60 mph and 65 mph became 100 km/h and 110 km/h respectively. The 50 mph limit in New South Wales became a limit of 80 km/h.

New South Wales introduced an absolute speed limit of 100 km/h in 1979, replacing the limit of 80 km/h The Northern Territory introduced an absolute speed limit of 110 km/h in 2007, along with 130 km/h zones on the Territory's four major highways.

NT open speed limits
The Northern Territory had no blanket speed limits outside major towns until January 2007, when a general rural speed limit of 110 km/h was introduced, although four major highways had higher 130 km/h zones. Speed-limit advocates note that the per-capita fatality rate in 2006 was the highest in the OECD and twice the Australian average. In 2009, the opposition (Country Liberal Party) unsuccessfully sought the removal of the 130 km/h limits on three out of the four highways where it applied, arguing that total fatalities in the Northern Territory had increased significantly during the first two years of the speed limit. In argument against the motion, the government provided more detailed statistics than normally published; these statistics showed a reduction in fatalities along the highways where 130 km/h limits were introduced. In 2011 the opposition argued for a return to "open speed limits" . For the 2012 election the Country Liberals' transport policy promised an evidence-based approach. After winning government, de-restriction of Stuart Highway was proposed; a planned 12-month de-restriction was initiated on 1 February 2014. The trial on 200 km of Stuart Highway was expanded later in the year to another 72 km, and continued indefinitely in January 2015 during a review of the initial results since "in the first 11 months, there were no recorded fatalities." From September 2015, a 336 km stretch of Stuart Highway between Barrow Creek and Alice Springs had speed limits removed for a 12-month trial. 130 km/h speed limits were restored on 20 November 2016 due to the electoral loss of the Country Liberal Party.

Derestriction signs in NSW
Often the start of rural default 'limits' or prima facie allowances were signalled by use of the speed derestriction sign, catalogued R4-2 in AS1742.4. (2009 edition has dropped from reference the R4-2 speed derestriction). The speed derestriction sign (//) had developed 'different meaning' over time at state and territory level, although its contract-meaning under Vienna Convention on Road Signs and Signals where the sign is catalogued C17a is "End of all local prohibitions imposed on moving vehicles" and has never changed. In the Northern Territory, they designated the end of speed restrictions. In Victoria and Western Australia they meant that the rural default speed limit applied, whilst in New South Wales, they indicated that the prima facie 50 mph limit applied. New South Wales's prima facie 50 mph limit, often signed by derestriction signs, was only enforced in cases where a driver's speed could be demonstrated to be excessive or dangerous in the context of prevailing road conditions. This was somewhat similar in principle to "reasonable and prudent" limits in other jurisdictions. This led to the widespread but misleading belief that no limit applied, and that derestriction signs indicated an "unlimited" limit. This belief, coupled with repeated studies showing 85th percentile speeds in excess of 120 km/h on major routes, comparatively high road tolls, difficulty in prosecuting speeding offences, and the variance in meaning of the derestriction sign across states, led New South Wales to harmonise its rural default limit to 100 km/h in 1978. The use of derestriction signs in New South Wales was officially discouraged, and on state controlled routes, 100 km/h signs were progressively used instead.