More drivers are letting their car insurance lapse because of the sour economy, putting themselves and others at risk.

Several hundred thousand drivers dropped their insurance in the past year as the jobless rate climbed, estimates a study to be released next month by the Insurance Research Council, an industry-funded group. Online agency Insurance.com says it also is seeing evidence recently of more uninsured motorists. It says that as many as 40% of callers following up on online applications had let their previous policies...