Not capitalized:
* I walked down the road.
* They drove on a dirt road.
* The road was closed for repairs.
Capitalized:
* Route 66 (part of a proper noun)
* Sunset Boulevard (part of a proper noun)
* Main Street (part of a proper noun)
Exceptions:
* Sometimes "Road" is capitalized when it's used as part of a street address, but this is less common:
* 123 Main Road
The best practice is to avoid capitalizing "road" unless it's part of a proper noun.