Highway driving is supposed to be "easiest" problem for self driving cars to solve, since there are fewer edge cases, less turning, etc., but it's also the most dangerous type of driving. You are much more likely to die going at 65mph than 25mph.

I think deploying self driving cars at <=25mph speeds at first would be wise. Personally, I wouldn't risk letting a car take over at high speeds until there is a longer track record of safety.