The braking distance is the distance that all vehicles are expected to maintain from the one ahead, in order to stop, if required, without running into its back.
When determining braking distance several factors should be taken into account: the alertness of the driver; the vehicle’s type and efficiency; speed; visibility and weather conditions; traffic; the slope of the road and the load on the vehicle.
In theory, calculating the right braking distance would refer only to the distance covered in one second, that is to say in the time generally needed to decide whether the stop lights flashing on the vehicle ahead require you to brake and to act accordingly, considering that the vehicle ahead has the same braking distance as the vehicle behind. What is the correct braking distance?
As speed doubles, braking space quadruples, therefore it would be wise to always keep the following braking distances (also when starting to overtake, that is when you pull out of the slipstream of the car ahead):

Km/h – minimum braking distance in meters – corresponding to
50        25                                   length of two buses
90        40                                   more than two articulated lorries
130       130                                 a soccer field

If the brakes are not perfectly efficient, the tires are worn out, or the vehicle is heavily loaded, the braking space increases considerably and you will need to increase it at least by half to drive safely.
These values do not apply in case of fog, when other considerations come into play.