Say Nothing: A True Story of Murder and Memory in Northern Ireland by Patrick Radden Keefe:
The car bomb, which was first introduced to the conflict in early 1972, represented a terrifying departure, because up to that point the size of most bombs had been limited by the sheer weight of explosives that a few paramilitaries could carry. Hiding the bomb inside an automobile meant that you could prepare a massive payload, then simply drive the device to the target and walk away. Whereas a suitcase or a plastic bag left in a busy shop might attract attention, cars were the perfect camouflage, because they were everywhere. “The car bomb provided an efficient container and an efficient delivery system,” Seán Mac Stíofáin wrote in 1975. “It yielded far greater administrative, industrial and economic damage for a given operation. And it required fewer volunteers to place it on the target.” In the streets of Belfast, an empty, unattended car became, all by itself, a source of terror that could prompt people to flee the area and authorities to descend, whether the car actually contained a bomb or not.