Because the stopping distance in metres – in good road surface conditions – is lower than the speed in km/h, a good rule of thumb is to drive at least as many metres behind the vehicle in front of you as your speed in km/h.
For example, if you drive at 90 km/h you should drive at least 90 metres behind the vehicle in front of you. At 90 km/h the stopping distance at ideal conditions is 67.5 metres, which gives you a margin of 22.5 metres.
This rule of thumb only apply if visibility and road surface conditions are good and if your car is in good condition. If you are driving a larger vehicle or towing a trailer, or if conditions are anything but ideal you need to increase your following distance even more.