Asphalt roads require periodic cycles of seal coating, typically on a 5 year cycle in the Pacific Northwest, to ensure a long useful life of 20-30 years. Temperature variations, precipitation, root intrusion, heavy vehicle, etc, all have an impact on how an asphalt road ages and wears with time.
Typically when an asphalt road is over 20-25 years old a community will need to look at an overlay or more costly full replacement of the road; which one is most appropriate depend on the condition of the road at that time. Skipping seal coat cycles earlier in the roads life is a common reason asphalt roads require more costly full removal and replacement later in its useful life due to significant cracking and subsurface drainage issues.
When funding for asphalt roads the reserve study will have a timeline of costs related to seal coats and overlays. If the asphalt appears to be deteriorating too rapidly or there are large cracks, drainage issues, potholes, etc. a community will likely need to fund for full replacement of the asphalt road instead of an overlay and should incorporate the higher expense into future updates to the reserve study. Full replacement of the road can be due to a variety of factors including improper installation, poor drainage, root intrusion, heavier traffic volume than originally designed for, etc.
*Reserve Analyst Tip* - When having a seal coat completed make sure you request and the vendor applies two coats of seal coat which is recommended by most manufacturers to achieve a 5 year useful life. When a new road is created or an overlay is completed refrain from seal coating for one year to let the gases in the asphalt road diminish which will help to extend its useful life.
Link: Asphalt Maintenance Manual & Video