I think it depends on the kid and your situation.
Basically... would I do it at Norfolk Botanical Gardens??? HELL no. First, you can see them from behind, so that's one reason... Second, there are some crazy people that drive through that one and some years I've literally thought it a miracle we didn't get hit.
Now, there are some other light shows where I would here... but it would still depend on the time of day going.
You know the area and the traffic, along with how patient your child is about long rides in the dark (aka... if not seeing much of the lights), and that's what has to be factored.
if the risks of being hit are none (save an almost impossible to happen freak occurance), I'd be fine with it.
But we did drive Ruthie's first light show with her RF at about that age (she was under 2... she turned 2 that March)... and not only did she NOT care... she fell asleep before we even got in to the lights... it was a LONG LONG line. Fortunately, I'd brought popcorn and christmas cookies, along with thermoses of hot cocoa for sippy cups (hey, Damian was fine to drink out of one to keep my car from getting hot cocoa on it) and mugs (for the grownups)
Had she woken up, though, because I didn't feel it was safe to turn her for it, I did have some toys along... that light up, in fact... that the brother sitting next to her in the third row could make sure were on for her to play with... because I did feel that it was safe enough to break our "no hard toys that could be projectiles" rule.
Ironically, she fell asleep the next year when she was too big to rf, too.
Last year was the first year she stayed awake!
SO maybe the answer is to go at a time where he will doze off?