speed bump

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

• noun An artificial ridge set crosswise into the surface of a street, parking lot, or driveway to make the operators of vehicles drive at a slow speed.

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

• noun A traffic calming measure, consisting of a transverse ridge in the road, to limit the speed of vehicles.

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

• noun a hindrance to speeding created by a crosswise ridge in the surface of a roadway