How Exactly Are Speed Limits Calculated?

The bane of many a driver's day, speed limits are there for reason. But how are they determined?
Christopher McFadden

Whatever your views on speed limits, you might have found yourself wondering how exactly they are determined. As it happens the process is pretty logical. 

At least on most occasions. 

Here we explore the history of speed limits and take a quick look at how they are set by local authorities. 


When were speed limits first introduced?

You might be interested to know, that speed limits actually pre-date the motorized vehicles. One of the first in history was set by the American colony of New Amsterdam in the 17th Century.

This limit prevented wagon, carts, and sleighs to be run, rode or driven at a gallop under punishment of "two pounds flemish" (about $50 today). In around 1832, "The Stage Carriage Act" was introduced in the UK to prevent "furious driving".

The rise of the locomotive led to further speed limits being imposed in places like the UK through the late 1800s. One of these, the 1865 "Red Flag Act" set limits of 16 km/h (10 mph) on open roads, 3.2 km/h (2 mph) in towns and 6.4 km/h (4 mph) in rural areas for mechanically propelled vehicles. 

how speed limits are set
Source: moppet65535/Flickr

Interestingly, the first person to be convicted of a speeding offense is widely considered to be one Walter Arnold of East Peckham, Kent, UK. In 1896, he was fined 1 shilling plus legal costs for exceeding a speed limit of 13 km/h (8 mph ). 

Many countries around the world began to follow suit, with an early speed limit being set in the early 1900s in Connecticut. Other U.S. states soon imposed their own over the next few decades. 

How are speed limits determined?

In short, speed limits are set by a public authority on the recommendation of opinions of the members of public and statistical work by traffic engineers. But, of course, there is a bit more to it than that. 

Road speed limits are set through a combination of politics and science. At the political level, speed limits are usually enforceable laws in many countries around the world. 

For this reason, they tend to be set by elected officials and enforced by law enforcement. Speed limits vary depending on their location (residential area or highway, for example) and other conditions.

In the United States, for example, city streets tend to be set at 25 mph, country roads tend to have speed limits of 50 mph and freeways tend to have speed limits of 60 mph. Similar limits are set in other countries, like the UK. 

While these limits tend to be set, in the U.S. at a Federal level, local officials can adjust them as required at their discretion. In places like Whatcom County, Washington in the States, statutory speed limits on country roads are 35 mph, not 50 mph. 

But there is more to determining and setting speed limits than that. In the vast majority of occasions, speed limits are set after a consultation period with the public and experts in the field.

Whether it is a newly constructed road or a request has been received to alter a speed limit, the process tends to go as follows. 

Members of the public are consulted in order to ascertain their views on the subject. This usually results in a spread of opinions from too fast (for people who tend to get tickets), to too-fast for residents who live, or work, at the particular location in question. 

Council members will then liaise with their jurisdiction's traffic engineers to access the speed limit of the road. They will take a more objective approach to formulate a recommended speed limit.

They need to balance the need to make roads as safe as possible while also enabling traffic to move as quickly as possible on a road. In order to help with this process, traffic engineers will conduct a speed study.

These consist of a review of the function, design and actual day to day use of the road. They will tend to ask questions like:- 

  • Is the road used by commuters or locals?
  • How wide are the lanes?
  • How wide are the shoulders?
  • Is the road curvy or straight?
  • Any intersections? Any blind spots?

Engineers will then try to work out the "85th percentile speed" for a particular stretch of road.

What is the "85th percentile speed"?

With a basic review of the road complete, engineers will then conduct a traffic count to determine the actual quantity and average speed of vehicles on the road. This will tend to involve laying a special cable across the road, or radar system, at strategic points to automate the process.

With this data, engineers will crunch the numbers to determine the "85th percentile speed". If the road is well designed, and an appropriate speed limit is already set, the "85th percentile speed" should be pretty spot on. 

The "85th percentile speed" is that in which 85% of all traffic tends to travel at or below a particular speed at the studied point on a road.

Engineers will also take a view of the historical rate of collisions on the road. This is then compared to the overall rate of the locality.

These tend to be calculated as a number of crashes per million miles traveled, but other metrics are also used.

With all this information collated and analyzed, the traffic engineer will then make a recommendation. They will usually make one of three recommendations depending on the outcome of their study: -

  • If crash rates for the road are high and the "85th percentile speed" is near an existing one, the engineer usually recommends a speed limit reduction.
  • If the crash rate is high but the "85th percentile speed" is a lot higher than the current speed limit, the engineer will tend to keep the existing one but increase law enforcement. 
  • If crash rates are low and the "85th percentile speed" is much higher than the existing speed limit, the engineer may recommend increasing the speed limit. 

What are the 3 types of speed limits? 

In the United States, there are 3 sets or types of a speed limit that you should be aware of. According to, they are as follows:- 

  • Absolute speed limit — The absolute speed limit is the posted speed limit in an area. You break the absolute speed limit if you travel over the posted speed by any amount.
  • Presumed speed limit — The presumed speed limit is not as straightforward. If an officer believes that you have broken the presumed speed limit, he or she believes that your speed did not suit the conditions at the time.
  • Basic speed limit — When an issue arises involving the basic speed limit, it means that an officer believes that your driving speed was unsafe even if it was at or below the posted limit. In this case, the officer would need to prove that your speed posed a safety issue.
how are speed limits changed
Source: Utah Department of Transport/Flickr

What is the speed limit when there is no posted limit?

In many countries around the world, there are set national speed limits for certain types of roads. This is the case even if there are no actual posted speed limits present.

In the United Kingdom, for example, national speed limits are as follows: -

  • Residential areas - Speed limits, if no signs are present, are set to 48 km/h (30 mph). The presence of lamposts, and houses, of course, is a good indication of this limit. In some circumstances, the speed limit might be 32 km/h (20 mph), but this should be clearly signposted.
  • A-roads: A-roads are the main fast single or dual carriageway roads. If the road is a single lane, the national speed limit will be 97 km/h (60 mph).
  • Motorways: The speed limit on motorways is also 113 km/h (70 mph). Again, some motorways, or certain stretches of motorway, might have lower speed limits, but this will be clearly signposted.

But these limits do vary widely around the world. 

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