Bonuses are the another source of money besides the normal income by driving. Unlike the normal income, bonuses do not depend on the type of car but they depend on the speed. The closer you go at the speed limit, the higher the bonus.
Bonuses are not paid out directly. The money goes into a pot that will be paid out at fixed trip-distances (10km, 20km, 30km, 40km, 50km, 75km, 100km... each 50km ... 1,000km). The current pot is shown in a display at the top of the screen. Whenever a payout occurs, a certain amount will be doubled and taken into the next pot, that means that doing longer trips will yield more money than driving the same distance in multiple shorter trips.
Collecting bonus money
Every 5 seconds, 1€ will go into the pot, depending on your speed. If you're close to the speed-limit it will be the full Euro. If you're too slow or too fast it will be less, maybe only 0.8 or or even less. (Though only full Euros are shown, fractional numbers will be summed up correctly)
The pot shows 100€
You will be rewarded 100€
A certain percentage (i.e. 30%) will be taken over into the next pot, in this case, 30€
The next pot starts at 30€ (rather than zero as before) and so on
The actual takeover-percentage will also increase the longer the trip is.