What rules are used when keeping the appropriate distance between two vehicles

You must drive a safe distance behind a vehicle travelling in front of you. Following another vehicle too closely is known as tailgating. If you tailgate you may not be able to safely stop in time to avoid a collision.

You should drive at least 2 seconds behind the vehicle in front during ideal conditions.

Add 1 second extra for each 3m of trailer length—when driving a vehicle towing a trailer or caravan.  

Extra points:  

1. Double your following distance in poor conditions

2. Increase following distances if you're driving a heavy vehicle.

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