Is a prime coat necessary?

At one time it was thought that a prime coat was an essential element of good pavement construction. However, in recent years some engineers have eliminated the use of a prime, especially when asphalt layer(s) (surface and/or base) is 4 inches or more in thickness. In many instances, prime coats have not been used even when surface thickness have been as thin as 2 inches. Over the past 20 years, few, if any, pavement failures can be attributed to the lack of prime coat.