|Average acceleration| = (change in speed) / (time for the change) .
For the biker:
|Acceleration| = (8 - 0) / (3) = 1-2/3 m/s³
For the car:
|Acceleration| = (30 - 0) / (8) = 3-3/4 m/s³
The magnitude of the car's acceleration is 2-1/4 times
that of the biker ... pretty substantially greater.