Many fans of motorsport agrees that rally drivers are the most talented drivers in the world because they must master every weather condition and every road surface. But they key to their success is their co-driver. Rally drivers cannot practice the course and must rely on a navigator or co-driver to survive. The co-driver uses a computerised odometer along with a supplied route book to communicate to the driver what lies ahead on the road. The route book describes in detail the road ahead and includes warnings for hazards such as cliffs, trees and junctions. Rally drivers determine what speed and angle to enter each turn or crest in the road by listening to their co-driver's constant instructions.