Many nations have adopted a form of a basic speed law, which states that motorists should travel at a rate of speed which is consistent with the driving conditions. This is not designed to replace posted speed limits, but merely to reflect the fact that safe rates of speed change depending on driving conditions such as ice, snow, rain, construction, or fog. Essentially, a basic speed law asks motorists to use common sense as a guideline, in addition to the posted speed limits.
Many nations post speed limits which indicate the maximum rate of speed at which a motorist may travel. In some cases, a national speed limit is established as a basic guideline, and individual states, provinces, or communities refine the speed limit to suit their needs. Common speed limits include a low limit for downtown driving, a higher one for highway driving, and a maximum speed for freeways and throughways. If no speed limit is posted in a nation where speed laws exist, the prevailing national law should be used as a guideline: for example, if the motorist is driving on a two lane highway, the national speed limit for a highway should be complied with.
However, a speed limit is only a basic guideline: in fact, traffic scientists only expect 80% of vehicles to comply with a posted speed limit, and sometimes less depending on the region. Engineers determine speed limits by looking at the type of road, where the road is located, and how the road is being used. Given experience with similar roads, a speed limit is set and enforced by law enforcement as needed. A basic speed law helps to supplement posted signs.
Most nations which have speed limit laws also have a basic speed law. Wording of these laws varies, but essentially it states that a motorist should not travel at an unsafe rate of speed given prevailing traffic conditions such as visibility, number of cars on the road, weather, or the type of road. In many cases, a basic speed law will also add that no driver should drive at a rate which would be considered reckless. Reckless driving is usually defined as a driving in a way which endangers life or property.
What a basic speed law means is that someone who is, for example, traveling at a posted speed limit during foggy weather could receive a speeding ticket. The citing officer could argue that the posted speed limit was not a safe rate of speed for the driving conditions. On the flip side, someone who was exceeding the posted speed limit on an open highway with no traffic and clear visibility could successfully beat a speeding ticket by arguing that the rate of speed was perfectly safe.