An advisory speed limit is a speed recommendation by a governing body, used when it may be non-obvious to the driver that the safe speed is below the legal speed. It is a posting which either approximates the Basic Speed Law or rule or is based on a maximum g-force exerted at a specific speed. Advisory speed limits are often set in areas with many pedestrians, such as in city centers and outside schools, and on difficult stretches of roads, such as on tight corners or through roadworks. While travelling above the advisory speed limit is not illegal per se, it may be negligence per se and liability for any collisions that occur as a result of traveling above the limit can be placed partially or entirely on the person exceeding the advisory speed limit. Signposting of advisory speed limits varies from country to country; Australia makes extensive use of advisory speed limits across its highway networks while the Richtgeschwindigkeit in Germany is valid for the whole autobahn network, while the United States and the United Kingdom only give advisory speed limits for hazards such as bends.
Use
Use of advisory speed limits varies by locale, but they are generally used to reduce speed along short stretches of dangerous road, such as on the tight curves of an off-ramp or on a busy shopping street. The advisory speed limit when not posted is generally the same as the mandatory speed limit in ideal conditions.
Australia
In Australia, if a person is involved in a single vehicle accident and the resulting investigation reveals that the driver was exceeding the Advisory Speed Limit displayed it can be a breach of the Insurance Cover Contract, resulting in no payout. Advisory speeds in corners are set out in AS 1742.2-2009 Manual of Uniform Traffic Control Devices, Part 2, appendix F. Corners are signed to indicate a speed at which lateral g-forces will not exceed 0.22g to fall well within the minimum suggested static rollover threshold of 0.35g for non-dangerous goods.
Germany
The Richtgeschwindigkeit is a legal term in Germany describing the advisory speed limit for roads without a mandatory speed limit. Autobahns have an advisory speed limit of on non-signposted sections. Exceeding the advised speed is not a criminal offense, but may result in greater liability in the case of a collision due to an increased danger of operating the vehicle. In Germany, the Autobahn-Richtgeschwindigkeits-Verordnung, introduced in 1974, recommends a speed of no more than for autobahns and similar roads, whose lanes are separated by a median or which have at least two lanes per direction, provided there are no traffic signs posting a lower speed limit. Until 31 August 2009, a different reference speed could be posted by the traffic signs number 380 and 381, according to §42 of the Germantraffic code, as seen above. As these traffic signs were only rarely used, they have been abolished, and will be fully removed by 31 October 2022.
New Zealand
Advisory speeds are not legal speed limits. Advisory speeds end in 5 to avoid confusion with mandatory speed limits which end in 0. The limits are established at 0.22g lateral g-force to fall well within the minimum static rollover threshold of 0.35g.
In the United Kingdom, most speed limits imposed by variable-message signs are advisory, and there are no sanctions for drivers who exceed them; a notable exception being the Gatsometer-camera enforced, MIDAS and ATM variable limits on the M25, M42 and M6 motorways. Crucially, the signs imposing these limits are distinct from regular, advisory VMS displays by the inclusion of a red ring surround, effectively changing them from advance hazard warnings into standard, mandatory speed-limit signs. As local councils require consent from the Department for Transport before changing speed limits, some use advisory speed limits on roads that the DfT refuses to officially downgrade. The usefulness of advisory speed limits has been questioned by a number of studies: one group from the Transportation Research Board found advisory speed limits through roadworks being consistently flouted by motorists, while an investigation by Manchester Evening News found that almost all buses in Manchester city centre exceeded the local advisory speed limit; some by as much as.
Signage
The signage for advisory speed limits is not defined by the Vienna Convention on Road Signs and Signals, and is therefore not standardised internationally. The United States uses a small yellow sign under the main warning sign, as well as a standalone variation on the standard speed limit sign, with a yellow background instead of a white one, the words "speed limit" omitted and an additional panel stating the type of hazard ahead. Though they list speeds, the U.S. advisory speed signs are classified as warning signs, not regulatory signs, as primary speed signs are. Australia uses a similar design as the U.S. in spite of regulatory speed limit signs being quite different. Germany used a square sign with a blue background and white lettering, similar to the minimum speed limit sign, and New Zealand uses a yellow background with black lettering. The United Kingdom currently uses an oblong white rectangle with black lettering stating "Max Speed".
