Distinctly different than the "speed bumps" found in many private parking lots, speed humps, speed tables, speed cushions, and raised crosswalks are raised areas of pavement placed across a road, engineered to safely moderate the speed of traffic.  Typically ranging between 3 and 4 inches in height and 12 to 22 feet long (in the direction of travel), they are often placed in series or intermixed with other traffic calming measures to provide continuous speed control along a corridor.  A popular and effective means of slowing vehicles, these devices typically reduce average speed by 4 to 7 MPH, and reduce the frequency and severity of collisions by 11 to 45%.  There can be a slight increase of noise in the immediate area of their installation.  One variation - the speed cushion is designed with gaps that allow emergency vehicles to straddle the device, thereby reducing or eliminating the 1 to 9 seconds of delay generally associated with speed humps.  Raised crosswalks, constructed like a speed table, are marked with high visibility crosswalk markings, and can be found at mid-block locations where speed control is an issue.  They not only slow traffic, but also make pedestrians, especially small children, more noticeable to drivers.