time to restart their engine, usually 30 seconds to a minute, and must be able to move. If the car cannot start and move, the driver is forced to concede and is eliminated from the heat. Most events require a slat of wood with a flag attached to be placed in the front column of the driver door, a judge breaks this to disqualify a driver, or a driver can break it in order to resign. The first and second place finishers of a heat move on to the next heat, in competitions with multiple heats. Depending on the sanctioning body, it may be illegal for multiple cars to collaborate and gang up on opposing cars in a sandwich effort, and could result in disqualification for both parties that do so; although the enforcement of this rule varies widely. The time it takes for one of these events varies, depending on if the derby being run has multiple heats and feature, or just a single heat, as well as the types of vehicles being demolished (compact car derbies tend to be finished much more quickly than school bus derbies, for example). Normal heats can last anywhere from 10–30 minutes, while an entire event can take place over the span of a few days. The last running car that makes contact with another driver wins the event. In addition to a winner, most derbies also award a "Best in Show" or "Mad Dog" award to the participant who puts on the most exciting or spectacular performance without winning the derby; this is usually decided by voice vote of the audience. (This is especially true in multiple-heat contests, where the addition of best-in-show provides more contestants for the feature event.) Rollover competitions Also included at some demolition derbies in the US and UK are rollover competitions, where the object is to drive a car so that only the wheels on one side hit a ramp, causing the vehicle to roll over repeatedly. Drivers take multiple runs at the ramp until their vehicle dies. The driver who completes the most rollovers before their vehicle ceases to function is declared the winner. Compact cars, especially hatchbacks, are used in rollover competitions. Their lighter weight enables them to roll more easily than larger vehicles. However, with modern high-horsepower unibody sedans and coupes now appearing on salvage lots, some of this conventional wisdom is being questioned