There travelling along a road is conditioned by the speed with which this can actually be crossed: this value can vary and we will generally have to deal with ever higher limits to the grow in importance. However, there are threshold values and technical considerations that lead to the correct development of road signs along the route, such as the visibility. Let’s understand together in this article what are the design parameters and technical requirements that lead to the definition of speed limits on a generic road layout.
The Highway Code and maximum speed limits
The Traffic Laws to theArticle 142 clearly reports the reference speed limits for road traffic. As expressly indicated by the Code, these limits implicitly define safety conditions for the protection of human life. What are we talking about? These are the maximum speed values at which it is possible to travel based on the type of road we are travelling on, that is, its importance:
- The limit minimum of 50 km/h is associated with roads in built-up areas;
- The limit maximum of 130 km/h is associated with motorways, which are the most important road infrastructures in our country.
Where do these limits come from? The aforementioned regulations do not provide any indication in this regard. However, it seems plausible to consider these numbers as the result of statistical analyses associated with the possibility of an accident and its severity. Some scientific studies estimate a 3% increase in accident risk for a 1 km/h increase in speed. The mathematical relationship that links the risk of an accident to speed is more than linear: it means that an increase in speed corresponds to a non-proportional, but more than proportional, increase in risk. However, other aspects can influence the choice of these limit values:
- Problems related to phenomena of noise pollution.
- Problems related to phenomena of environmental pollution.
In fact, speed has a significant impact on the CO production2 produced by combustion engines, as well as on the levels of noise that propagate into the environment. The decision is therefore a compromise condition that encompasses different technical aspects, with a significant weight associated with driver safety. Is that all? Of course not! The planimetric-altimetric development of the route and the interaction that this has with its surroundings are elements that influence the actual speed of travel of the infrastructure. Let’s see the fundamental points.
Safety associated with the physics of the vehicle in motion
Various physical phenomena that characterize the vehicle in motion are linked to the speed of travel. A vehicle moves along the road axis for rolling, that is, without there being any relative slip between the wheels and the road. This happens because at the road-wheel interface the adhesion force. However, if this fails for some reason, the rolling motion is added to (or replaced by) the sliding motion and the safety conditions are drastically reduced: the vehicle “skids“.
But when does adhesion decrease?
- This force decreases as the speed increases, because the contact capacity between the wheel and the road surface is reduced. Another similar situation occurs when water is present, or when the famous surface water film (aquaplaning) which is the cause of reduction surface roughness and consequent reduction in grip.
- Similarly, a vehicle in curve is subjected to additional forces during motion, in particular the centrifugal force that tends to push it out of the curve. This force, which is normally counteracted by grip, increases with the speed of the vehicle. When cornering, therefore, we find a vehicle in which grip – which decreases with speed – is more engaged than on a straight line and, in addition, the stressing force increases as speed increases.
In essence, the higher the speed, the less safe the vehicle becomes while moving. This is one of the reasons why, along the route, we find different speed limits: we notice that the limit is reduced in case of rain, or it could be reduced near a curve with a relatively small radius.
Visibility issues
The presence of a correct visibility for the driver it represents another aspect of fundamental importance in defining the right driving speeds. the driver must have the necessary time to notice the presence of an obstacle, of any nature, and stop the vehicle safely. But how is visibility linked to speed? Given a distance from a given obstacle, I must ensure that the speed is such as to allow the driver to brake in time, stopping before the obstacle.
The speed limit can therefore be influenced by the presence of obstacles outside the carriageway, but which nevertheless affect the driver’s visibility. Not only that: the presence of speed bumps or bags in the altimetric profile of the road can affect visibility, generating a visual phenomenon in which one can have a partial loss of visibility on a part of the trackThe distance that affects the stretch of road for which information is lost is called reappearance distance and increases as the speed increases: this means that the faster I go, the more information I potentially lose on the track if there is a gap in the altimetric profile.
How to set speed limits: the design speed diagram
How can all these aspects be taken into account? The road project requires the preparation of a project document known as velocity diagram. It represents a fundamental document because it allows the tracking of the project speeds on the entire planimetric development of the work, taking into account the presence of straights, curves and visibility. Through this diagram it is possible to define the actual travel speeds and guarantee, through appropriate road signs, the right safety of use of the infrastructure.
Control through Tutors
Respecting speed limits is a safety prerogative. However, users of the service do not always behave in a manner that is appropriate and respectful of these limits. Control tools are therefore needed to ensure compliance with road safety. There are different types. The most famous are certainly speed cameras, which measure the instantaneous speed of vehicles in motion. However, they have a big problem: by carrying out a specific check, they do not ensure compliance with the limit along the route, unless many of them are installed.
The solution to this problem is introduced on the motorways through the system Tutor. This innovative system is developed by Autostrade per l’Italia and already in the first year of installation it has contributed to reducing the accident mortality rate by 51% on the network. Why is it a more effective control mechanism? Because it does not measure instantaneous speed, but average. In essence, the system calculates the average travel speed (speed = distance/time) of a given section between two consecutive devices by performing a measure of travel time of the generic vehicle trait. We can also see the whole thing as an application of the mathematical theorem of the integral mean: there will certainly be, in the time traveled, an instant for which the speed of the vehicle was higher than the limit, if the average itself exceeds this limit. In fact, it is as if there were a speed camera for each point of the route covered by tutor.
