I have heard that the bolt attaching the unit to the hitch can loosen as you drive long distances. Does the lock prevent this?

I can reiterate Damon’s answer, I love these bike racks and I have never had a single problem, and I have never had a problem with the bolts coming loose. With the proper lock washer you should be able to drive thousands of miles with it never coming loose. To me these are the best bikes money can buy, do not fear in purchasing a Thule product.

I have never heard of this happen if torqued on properly with a lock washer, thousands of miles over many years on mine. For that matter everybolt on your car would be a concern if that were the case. But I have to assume if someone removed the lock-washer from the bolt and didn't tighten the bolt, it could vibrate loose. The bonus is the rack is nice and tight and your bikes ride alot sturdier.