Firstly, you are supposed to drive at legal speeds on the highway and not speed. To answer your question, it's not only with DASH but SUKE elevated highway as well where the surface of the roads are uneven and undulating when it's supposed to be a uniform flat road surface. Good joke that the contractor purposely made the road surface uneven. The reality is it's mainly on the quality of the construction especially at the pililars where the joints between beams are poorly constructed, and it's usually this area the "bumps" exist. There are good contractors and not-so-good contractors, and some of the better contractors will be able to produce better quality work.