The easiest way would to be make the game object faster. Another way is to have the environment parallax faster and faster until you hit a pothole and just set the parallax speed back to the original speed. Or maybe a little speedometer to add more effect as well like they have a motor bike, turtle slow and rabbit fast and have a dial rotate towards how fast you are going; you could also just have a flat number based system as well showing the numbers incrementally get bigger to a max which would give the effect of going from slow to fast.