Question
QUESTION: We have a stand of western red cedar trees that are 8' to 12' in diameter along our property line.  The property next door is being panhandled into three lots and the driveway will be along the property line where the cedars are located.  How far away from our cedars should the driveway be located to keep from damaging their roots?

ANSWER: I assume that the diameter you are talking about is the width of the foliage not the trunk diameter. If so the western red cedar has a normal width of about 20-25 feet and the roots will extend about 10 feet beyond that so I would say the safe distance would be 25-35 feet away from the trees. This would also make sure the limbs do not reach the drive and scratch the cars as they drive by. I hope this answers the question you asked.

---------- FOLLOW-UP ----------

QUESTION: Ok, so I was a little off on the diameters.  They are really 8" to 12" as shown on the site map done for the lot approval process.  If the driveway is 3 feet (or potentially less) from the tree's trunk (the property line) could that damage them?  Is there anything thing we can do to keep them healthy?

Answer
If you dig down more then 12 inches to put the drive in this will damage the roots this close to the tree. The general rule is not to damage more than 25 % of the roots with digging. If through draw a circle extending around where the outer most branches are and than draw the cut through the circle where the digging will be you can estimate the % of the roots damaged. Three feet from the tree will take in large amount of the roots. Cedar roots are not a problem with up lifting driveways so if you can construct the drive with out digging more than 3-4 inches deep you should not have much problem with the root damage. After the drive is in I would suggest that the trees be fertilized with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. This will stimulate the remaining roots to grow better.