Ultrasonic motion detectors only measure the position of an object that is directly in front of it, within about 3 meters (10 feet). It is intended to measure the velocity of an object or person moving toward or away from the sensor. This works well for the relatively low velocities of a walking person or a cart rolling down a track, but our motion detectors are not practical for using with cars on a road or baseball pitches.
Measurements for faster objects such as these are easily done with a handheld radar gun; in fact the athletics department of a school may already own one for documenting baseball pitching speeds. You can browse http://www.stalkerradar.com/ for some available radar guns. A low end baseball radar gun is about $500.
There is an article on physics use of radar guns in the Feb 2002 TPT, page 94-95.