While Niantic doesn't specifically state if this is okay or not (that I know of). They have mentioned cheating in their Trainer Guidelines.
Methods of cheating […] include at a minimum the following: using modified or unofficial software; playing with multiple accounts (one account per player, please); sharing accounts; using tools or techniques to alter or falsify your location; or selling/trading accounts.
Also the game still works when driving at speed so it can be assumed that it is intended that this is okay. Considering that there is a speed lock on hatching eggs. If they intended that you couldn't play while driving (as passenger) then they would have put this same speed lock on catching Pokemon as well.
Also note that recently the scanning speed was reduced from 5 seconds to 10 seconds. Which means that at about 60kmph you will be covering half the area you would've previously. This means you're going to be finding much less Pokemon while travelling fast as the scanner will miss some.