Thanks!
The idea was that colliding with things will give you boost and that in order to keep running you would have to keep hitting things. Because sending things into the air is fun :)
I implemented a non-linear speed depletion and I wanted to make it so running at medium speed is what happens most and running at low speed is likely to end the game and that running at maximum speed occurs rarely.
I played with values and ended up with current system, where its a bit challenging at first to gain speed, but once you are at top speed - it gets easy. In order for the game to not be so boring I made items more scarce the more score you have, trying to make sure that once people get top speed they can easily run for time long enough to listen to full song :)
I wanted to implement repeatable achievements that would give temporary speed boost, something like "you hit 25 cars", so the player could strategize, but I ran out of time