A series of self-driving cars are driving along a road such that the distance between the cars is precisely the minimum recommended according to the formula:

Overall stopping distance (ft) = (Speed(mph)^2)/20 + Speed(mph)/2

So that at, for instance, at 40mph, the distance would be (40^2)/20 + 40/2 which is 100ft.

For simplicity let’s ignore the length of the cars themselves.

Two lanes that are each travelling at 60mph merge into a single lane. What speed will the cars in the merged lane be travelling at?