When paving material is first laid down, it has to be compacted (steamrolled). That's why, when you are told you are getting 2 1/2 inches, you need to know if that's 2 1/2 inches BEFORE or AFTER it gets squashed down to 1 inch, which often isn't enough. Ask your estimator. The answer you get should be AFTER compaction.

The same rules apply for almost every industry. There are paving outfits that roam around, promising cheap paving. They can get their prices down low by cutting corners. Using less material, not clarifying what "After Compaction" really means, paying their employees less money and using older equipment. The results aren't always what their clients pay for, but the results may only be discovered after a long, wet winter.
Everybody loves to save a buck, but when it ends up costing more in repairs or redo's later, it isn't much fun anymore.

These are a few questions to ask, and points to raise:

• Make sure the contractor is local. Local businesses are more accountable;
•  Research the contractor. BBB ratings, Yelp, star ratings and testimonials;
•  Get an estimate. Get everything in writing. Learn what 'After Compaction' means;
•  Ask questions. Who is doing the actual paving? Them, or somebody else?

One of the most important parts of your new asphalt surface is actually the sub base. 

The sub base provides a stable surface to support new pavement. 

The sub base is a frost barrier to help reduce winter damage due to freezing and thawing. 

During the installation, base thickness, base stability and compaction are important steps.

If the sub base is not appropriately compacted, the asphalt surface on top will not provide the years of durability you think you're buying

Full-depth asphalt driveways are built entirely of asphalt paving mixture - from the soil subgrade up.

Full-depth driveways keep water out of the pavement, so water never enters the pavement to swell when it freezes.

Full-depth asphalt provides a better balance of strength and flexibility - plus durability - than any other material.

For improved soil stability, it is recommended that topsoil containing clay be removed or modified. Soil stabilization or as much as 6 inches of crushed aggregate base may be required for some subgrade conditions.

Both the subgrade and aggregate base require thorough compaction to provide a solid foundation for your new driveway.

Once the site is ready, paving with asphalt follows. In many cases a 4-inch thickness may be adequate, but 5 or even 6 inches of full-depth asphalt will assure you of a stronger, stable driveway under a wider range of climate and loads.

As an option, some contractors use 6 to 8 inches of compacted aggregate, or gravel, as a base for 3 inches of asphalt pavement.

'Bumps' in asphalt concrete overlays can be extremely frustrating when they happen.

With more focus on ride quality and pavement smoothness, paving contractors, asphalt concrete providers, paver manufacturers, and roller manufacturers, engineering firms and agencies have all researched ways to assure a smooth finished overlay.

Pavement design, field conditions, paving and compaction equipment and general construction practices are all suspect and have been cited as factors for bumps in overlays. 

One construction practice that is in control of the contractor is compaction of the overlay. In the process of overlay compaction, the rollers tend to shove the mix forward.

If this happens on a job where the original pavement has 'uniform restraining characteristics, a smooth finished surface can be achieved.

However, if this original underlying pavement surface varies significantly due to irregularities or conflicting materials, uneven shoving can result in unwanted bumps.