The June 2 issue of The New Yorker has a fascinating profile of Charles Heal, a long-time expert on the development of "non-lethal weapons." The article first discusses various types of weapons designed to stop a person or group of persons without puncturing them: bean-bag guns, dogs, irritants like tear gas, malodorants, obscurants that interfere with sight (such as smoke), Tasers, flashbangs, soporifics, tanglefoam. The Los Angeles Sheriff's Department tried TigerLights, a combination of flashlight and pepper spray. The pros and cons of all these devices (including the Active Denial System made for the military, which was the subject of a 60 Minutes interview just last Sunday) are discussed.
The more interesting part of the article, however, comes when the author accompanies Heal on visits to people who have taken out patents on inventions they hope will be the next big thing in crime fighting, such as the Carpoon. This does exactly what you think: harpoons a car in high-speed chases. Heal points out that it's not feasible because it will cause both collateral accidents and lawsuits. Lots of lawsuits. One criterion for whether law enforcement agencies will invest in a new weapon is whether it will reduce law suits from the old ones. They are not interested in those that will create yet more lawsuits.
What's really needed for vehicular pursuits, Heal said, is "a directed-energy device that uses a signal from our car to interrupt the other car's ability to supply fuel or ignition. It may make the fuel mixture too rich or too thin, and if you can change it even briefly, the car will die. That's the Holy Grail. Whoever invents that will be rich from the day he does,"