Impulse = ft
As t increases, f decreases. That means that when cars go slower around school zones, its impact is less if it hits something. It's a safety mechanism employed so if school kids or whomever are hit by a car, their injuries are less severe than being hit by a car travelling at higher speeds.
Momentum = mv
Since the weight of the car is always constant, velocity is going to change. If the car is faster, it will have a higher momentum and force than a slower car. (Also note how changing momentum is equivalent to impulse if I remember correctly) so you want lower speeds because there will be a smaller impact when people are hit. Essentially, it's for pedestrian and travel safety. It's especially important in schools because the risk of injury is higher because children are more likely to run around, etc.
