Although thermal cracking, frost heaves, oxidation and vehicle damage all contribute to road deterioration a more common cause is the damage road maintenance causes to the bonding of the 'hogging'. The hogging is construction slang for the usually silica derived aggregate taking the 'live loads' (moving weight) pressure. When the bonding is disturbed it's structual integraty is undermined resulting in what is known as the 'bean bag effect'. Imagine a bag of frozen pea's and you sit on it, it will retain it's form but if you poke it with a stick areas of the frozen pea's bonding will start to give way or split the packet. Much like the shifting aggregate under the road splitting the Tarmac. A simple way of seeing this in action is by observing the road compared against the flow and direction of traffic. Heavy freight is usually heading into the capital on the left hand side resulting in more aggregate shifting over to the right causing more visible destruction often over a 10 to 20 year period. New Bitumen concrete based pavement is predicted to last about 30-40 years.
Lecture over but I'm a civil and demolitions engineer hoping to help.