It won't stop Covid.
But it will reduce the number of people from an infected area going to another area with lower infections.
Maybe say a 10 mile zone - unless you have a really good reason (such as it's the nearest food shop, hospital, GP - or caring for someone)
It should stop people going to pubs etc, shops from areas of high infection.
Our Tier 2 city has just gone to Tier 3. We had a lot of people from other cities come to our city over December.