Will Our Future Brains Be Smaller?

Who knows. We might continue to advance technologically, and end up with automated vehicles. On the other hand, civilization might collapse, and we might need those quick reflexes more than ever.
I would guess that animals that frequently cross streets are evolving in some way to deal with danger from that.

http://www.sciencedaily.com/releases/2008/07/080708200639.htm

ScienceDaily (July 8, 2008)
The speed at which we react to threatening situations can have life or death implications. In the more primitive past, it could have meant escaping a wild animal; today it might mean swerving to avoid a head-on car crash.
...
The results of their modelling showed that when the threat level is high, such as the risk of being attacked by a dangerous animal, it is very useful to have the fast-acting, if inaccurate, system. But when dealing with situations which don't occur very often, or complex scenarios with many conflicting cues such as social situations, the cortical system is of more use than the sub-cortical system.

Trimmer commented: "As life became more complex, the benefit of gathering information before making a decision put an evolutionary pressure on the early brain. This may have led to the rapid development of the cortex in mammals. So if humans continue to live in a world of dangers such as wild animals or fast-moving cars, there will still be an evolutionary benefit to maintaining the sub-cortical system, and it is unlikely to atrophy in future humans."