The safety distance from the vehicle travelling ahead depends on...

When travelling behind another vehicle, you must leave a gap that allows you to stop, in the event of sudden braking, without colliding with it, taking into account the speed and the conditions of grip and braking. Furthermore, if you are travelling without signalling your intention to overtake, this gap must also allow the vehicle behind you to overtake safely. For these purposes, drivers of vehicles with a maximum authorised mass of more than 3,500 kg and vehicles and vehicle combinations more than 10 metres in length must maintain a minimum separation of 50 metres. It is sufficient to maintain a distance to avoid a collision (it is not necessary to maintain a distance to allow overtaking): - In built-up areas. - Where overtaking is prohibited. - Where there is more than one lane in the same direction. - When traffic is saturated. - When you intend to overtake.