That is where being legal, and being safe can contradict. You merge onto the highway at the speed of traffic to be able to merge safely. The people coming onto the highway are the ones who must yield, not the highway traffic, therefore you are at the mercy of whatever speed they are. (these Utah fuckers don't grasp this concept though, they'll get up to 55 and wait till just before they have to merge to check their mirrors to see if it safe.... assuming they aren't texting on their phones.)
Anyhow, if traffic is doing 90, and speed limit is 70, you should be merging at 90mph. Both breaking the law, and doing as the law intends for the yielding traffic. Either way, I don't think I've ever seen someone pulled over for merging slower than traffic!