road hog

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A motorist whose vehicle overlaps the traffic lane used by another motorist.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A motorist who straddles two lanes of traffic or otherwise monopolizes the road.
• noun A bad or inconsiderate driver of a motor vehicle.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a driver who obstructs others