Car theft in the UK fell two-thirds from the mid-1990s as part of more widespread crime drops, and has been attributed to improved vehicle security. This study develops a Security Impact Assessment Tool (SIAT) to gauge the contribution of individual security devices and their combination. The metric of impact derived is termed the Security Protection Factor (SPF). Cars with central locking plus an electronic immobilizer, and often an alarm, are found to be 'SPF 25', that is, they were up to 25 times less likely to be stolen than those without security. That impact is greater than expected from the individual contributions of those devices, and is attributed to interaction effects. Tracking devices are found to be particularly effective but rarer. Protective effects were greater against theft of cars than against theft from cars or attempts, almost certainly reflecting the difficulty imposed on thieves by electronic immobilizers. It is suggested that this type of analysis could be usefully extended to other crime types and security combinations. The analysis also lends support to a 'security hypothesis' component of an explanation for the major national and international crime drops that is based in the criminologies of everyday life. (Published Abstract) Tables, figure, notes, and references