The experiments were a joint effort of the Tucson Fire Department fire investigators, Tucson Police Department Crime Lab Criminalists, and the Tucson Police Department Arson Detectives Unit. The highway flares examined use a friction ignition system. The fusee uses a two-part igniter; when the two surfaces are rubbed together, a flame is produced and the main composition is ignited. The experiments involved the construction of various scenarios that might explain the unintended ignition of the flares. The research group determined that it is common practice for officers at accident scenes that require flares to prepare them in advance by removing the protective caps from both the igniter tip and from the scratch surface in anticipation of the need for additional flares. This exposes the igniter tip and the scratch surface. If these exposed flares are not used, they are returned to the trunk of the patrol car with the caps removed, thus creating a possible scenario for a fire. A random inspection of the trunks of several patrol cars found that some fusees were uncapped, thus exposing the igniter tips; some of the protective covers from the scratch surfaces had also been removed. In addition, loose flares were observed in some trunks. Also, the box containing the fusees was stored improperly among equipment and supplies. Although the fire-cause investigation is continuing, the department has taken steps to ensure that fusees are properly stored in squad car trunks. 6 figures