What is it with these "upgrade games" where the only purpose of having upgrades is drag out an excessively simple 5-minute game for hours. Repeating all the levels over & over is REALLY boring. Also, that screen between levels is pointless and annoying.
Grinding isn't fun... and this game is terrible. Get rid of ALL the upgrades & instead make a score for 'total distance traveled' for replay value. It wouldn't make the game great, but it'd make it tolerable.