Several callers from past shows have suggested adding sensors to cars to detect when children are locked inside and in danger of suffering from heatstroke. Popular Mechanics has a story on just such a scheme being designed at GM in the September 2001 issue on page 77. GM describes the technology on its web site as well. It should be clear from reading this that the company is making a few assumptions that would make the system less than completely reliable, as Bill pointed out to the callers. In particular, note the area covered by the sensor and consider how likely people near the car are to recognize the alarm as something other than a normal car alarm, and then consider how many people ignore car alarms as it is.