Administrative distance is a critical factor in route selection as it indicates the trustworthiness of a route source. The lower the administrative distance value, the more preferred the route is. This factor is pivotal in environments where multiple routing protocols are used, determining which protocol's route is prioritized when different protocols provide route information for the same destination. Prefix length and metric are also important but serve different specific functions: prefix length delimits the network and subnet portions of an address, helping in summarizing routes, while metric is used internally within routing protocols to define the best route among multiple available paths based on hop count, bandwidth, or other criteria.