Conifer Trouble

Please see picture above.

I have a row of conifers at the front of the house which seperates the drives from our neighbours.They are about four years old and this season four of them in the middle of the row have developed brown branches/leaves,which have sudsequently died.I have cut the branches back but I am unsure of the reasons why this has happened.Any advice would be most welcome.

Cypress aphid can cause this. The RHS has a section on their website about brown patches on conifers. Areas that have died do not grow back, in my limited experience.

Gardening Grandma wrote (see)

... The RHS has a section on their website about brown patches on conifers. ...

The RHS page of advice is here:
http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=132

There are many possible causes. The section on prevention and control is here:
http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=132#section4

I have a large conifer in my garden. A few years ago several large areas of the tree went brown, and apparently died. I hadn't pruned or trimed the tree, so this was most likely caused by disease or an insect. I cut out all of the brown wood. The rest of the tree survived. Now, the tree looks perfectly healthy, apart from the fact that half the tree is missing, due to the huge chunks that I cut out...

Just a thought. If they are next to a drive, I suppose no noxious substance like oil has been spilt on their roots?  

I was thinking salt poisoning - from salting the driveway when icey, or run off salt from roads, or wind damage or both.