eli5: How do they decide what the speed limit is for a road?


Basically the title. I’m curious what they do to decide. surely they don’t just stick a random number on a sign and call it a day.

In: 16

Around here they use random signs. They will take an old rough road marked 45mph, widen and replace it, then lower the speed to 35mph

It depends largely on the nature and purpose of the road. Things like width, straightness, slope, signage and surface influence how safe it is to use that road at speed. Obviously you need to go slowly on a narrow, windy road if you don’t want to accidentally leave it. If you want a road to be fast, you need to make it straight enough that steering is easy and you need to make sure people have enough time, space and notice to react to whatever they’re approaching. The other thing is that you need to make sure the limit accounts for transitions onto other roadways or through areas of different usage. A smooth transition between different speed zones is better than, for instance, letting a highway exit directly into a school zone.


They monitor the average speed that drivers tend to travel on it based on their own sound judgement and then set the limit 25% lower than that in order to establish a revenue stream.

My turn! When a road is being designed they have to consider how the road curves either from changing elevation, switchback roads, or both. IIRC the speed limit is generally determined by how fast an object of X mass(the cars) can safely make the turn in best weather scenarios. Safely meaning you can make the turn at a certain speed without losing traction and driving off the road or still have time to slow down some in case of emergencies.

Most designs are also made with nighttime considerations so when your headlights are on and driving around those road curves we don’t blind each other. (Why it’s not legal to tamper with your headlights.) Good design practice should also drop the driving speed limit from the design speed limit to accounting for bad weather conditions. This still doesn’t say anything about road width, city/school zones, straightaways, etc. Initially we did not care so much about making roads but after the first decade of automobile accidents it was apparent changes needed to be made.

Many jurisdictions do surveys of the speeds actual traffic travels on a road before speed limit signs are placed, and set the limit at some legally defined point in the speed distribution.

The one I specifically remember was for a SF Bay Area city. Limits were set so that 85% of the traffic was measured to be below that speed, rounded to 5 mph. I knew this because locals had complained that the posted limits were too low, and the town was legally required to re-survey the traffic.

In the US, as with so many things, there is often a mix of city, county, state, and national laws that must be complied with.

Generally, you get the default speed limit and you design the roads to make that safe — you get minima for the width, radius of the curves and probably more things.

There are also rules for setting a specific speed limit. For a given road, when not explicitly constrained, people will naturally drive at some speed (or, rather, a distribution of speeds, because different people are different). You set the speed limit to the given quantile of that distribution.

Interestingly, in some places (I’ve seen this is Zürich), roads are designed to make that natural speed lower, by making them narrower and adding some curves (by narrowing them down _on alternating sides_).

It probably varies by country but at least in Portugal (and the whole EU?) there are speed limits defined by type of road. Within cities/villages = 50, Outside = 90, Highways 120. Then, based on specificities of roads they may set lower speeds. For example, next to schools = 30, roads where pedestrians are forbidden = 80, zones prone to accidents = 20 less than normal.

All the reasons others explained, and then sometimes there’s politics involved, where local parents want their streets slower so they don’t have to keep their kids out of the street. Or speed traps so cops can steal from anyone passing through.

There are guidelines that help with the settings of them. However things which are considered are the: geometry of the road, visibility, what kind of traffic is most likely to be on it (Small cars, heavy machinery, big trucks, farm equipment), how much traffic is there, what are potential hazards. From this an estimation is made by comparing it to roads with similar properties and how many accidents there are on it. There are different figures for this, but I know that in Finland the rule of thumb is that 10km/h reduction in speeds reduces the chance of an accident by 10% (The figure used isn’t exactly that, it was like 9km/h reduction reduces the accidents by 7,5% or about there).

Speed limits are not arbitrary, they are based on statistics and guidelines. There is a whole field of art and science to balancing out traffic flow and probability of accidents. But we know for a fact that if the speed limit is reduced (and people follow it) there will be less accidents and less deadly accidents.

They gradually increase the speed limit until a certain number of fatalities per 10k TH (traffic hours) are reached. Then they reduce it by 5mph. Basically the same way they determine lethal doses of medicine.