Tyres and Tarmac
Leaving a suitable gap between you and the vehicle in front when stopped has many advantages.

Make sure that you can see the tyres of the vehicle in front of you touching the tarmac when you have stopped.

This is safer if you are involved in a rear end shunt, as you may not be pushed into the vehicle in front of you.

If they stall or break down, you have room to manouver around them and continue, rather than being boxed in and maybe having to reverse on a busy road.

If an emergency vehicle approaches, you will have more room to manouver and allow it to pass.

Mirror - Signal - Manoeuver
It sounds simple, and it is.

How many people do you see who don't do this very simple thing, that should have been taught to drivers since taking their L test. It's part of the system of car control.

Slowing Down
Check your mirrors, then slow down

You should always consider vehicles behind you when you are slowing down.

Check your mirrors, then consider how much braking is appropriate.

Motorway Stopping Distance
How can you judge your stopping distance on a Motorway ?

Stopping distance (in good conditions) at 70mph is about 100m.

How can you judge that you have left at least 100m between you and the vehicle in front ?

The motorway marker posts alongside the hard shoulder os a motorway, are spaced every 100m.
You can use these posts to judge the distance between you and the vehicle in front.

As the vehicle in front of you passes a marker post, there should be at least one other marker post between you and that vehicle. If there isn't then you may be too close.