Acceleration = (change in speed) / (time for the change).
Change in speed = (15 - 30) = -15 km/hr
Time for the change = 4 sec
Acceleration = (-15 km/hr) / (4 sec) = -3.75 km/hr per second .
Is that a lot ? Not much ?
Let's convert it to a unit that we can think about:
(-15 km/hr) x (1,000 m/km) x (1 hr / 3,600 sec) =
(-15 x 1,000) / (3,600) = -(4 and 1/6) m/sec .
So the acceleration of the bus is -(4 and 1/6) m/sec² .
The negative sign means that it slowed down.
(4 and 1/6) m/sec² is about 42% of the acceleration of gravity ...
the acceleration the bus would have if it drove off of a cliff.
When the car or the bus you're riding in slows down at that rate,
you feel 42% of your weight pulling you forward against your
seat belt. That's quite a drastic acceleration !