Speed limits on large roads 0
By braikar, 09-05-2013 - 13:12 in Infrastructure, Roads

This applies to the Western industrialised society.

Speed limits applied to this day were limits decided around the mid 19′ for cars that do not exists any more. This simple fact is so obvious to me and yet I don’t hear anyone talk about that. Instead I see speed limits being reduced and reduced, more speed cameras installed everywhere. Most importantly a growing disapproval of these measures by the population that feels trapped and under excessive surveillance; it depends on countries though; Germany is the only country trying to find real useful solutions.

Bear in mind that the speed limits nowadays were ones decided some fifty years ago and that our cars have evolved to be more secure and able to work safely at higher speeds. Our engineering skills to build roads have also greatly improved; meaning straighter roads, a more uniform road surface etc.

The following ideas apply to large roads, highways/motorways.

We now have average speed cameras; multiple cameras placed at regular intervals along the roads to estimate our average speeds. However these are badly misused. Because nowadays a completely straight 3 lane road with a speed limit at 110 km/h has the same speed limits whether there is a lot of traffic of none at all! Only Germany and the Netherlands have variable speed limits depending on traffic density at working hours etc. It is implemented in different ways in both countries though, but none close to what I have in mind.

What would be useful and meet the approval of the population would be to use average speed cameras to automatically count the flow of cars on a specific stretch of road. Then for a specific flow; I don’t know the precise details; if there are:

  • more than 1000 cars per minute, speed limit = 90 km/h
  • more than 500 cars per minute, speed limit = 110 km/h
  • more than 300 cars per mintue, speed limit = 130 km/h
  • more than 100 cars per minute, speed limit = 150 km/h
  • less than 100 cars per minute, speed limit = 170 km/h

The cars density flow would also need to be adapted to the number of lanes on the road, the present climate conditions (rain, high wind, snow, low temperatures, etc.), also the quality of the road, the slope of the road and probably other details that could be useful to consider.

By using 170 km/h as an upper limit, I’m not promoting high speed limits, but I’m just taking into consideration the fact most cars are able to drive at these speeds quite safely. Though it wouldn’t be safe to drive at these speeds if the density of cars were really high and security distances between cars could not be maintained.

Hence, by following this idea, we would have average speed cameras used for controlling cars (and inflict penalties for ones not respecting the speed limits), but the speed limits would be properly matched to the density of the car flow and other details regarding these specific stretches of roads. There is no logic nowadays in having a speed limit at 110 km/h if the road has 3 lanes, is perfectly straight for 100 km and only a couple of cars on it… Yet these are common aberrations that I often see when on the roads.

Germany also has a good method of not limiting speeds for some stretches and limiting it when there is an entrance or exit, for which it is important to limit speeds when cars have to slow down to exit or accelerate to enter the road. This could be further improved upon, by setting different speeds for different lanes just for entrances/exists, so as to not disrupt all the flow, because often in Germany it goes from ‘no limits’ to 120 km/h, which sometimes is abusive and forces cars to break heavily… If there were 3 lanes it could be set at 150 km/h on the most inner lane, 130 km/h on the middle lane and 110 km/h on the lane that is on the side of the exit/entrance. Once again with automatic radars to measure the flow of cars these speeds could be adapted more precisely depending on density, road characteristics, climatic conditions etc. This could prove to be less disruptive to the car flow on exit/entrances.

Anyway, the main idea is:

Average speed cameras used to measure the density of cars flow to automatically adapt the speed limits; hence no more need to set static variable speed limits for specific hours and also giving the liberty to increase speed limits to much higher values when flow density is very low.