A lack of visibility regulation is the biggest problem in current vehicle design in my opinion. A 5' (152 cm) tall driver should be able to clearly see a small child standing a few feet away in front of or to either side of any vehicle allowed on public streets. Any vehicle modification compromising that should be strictly prohibited.

Steep financial liability for pedestrian/cyclist accidents for not only the driver but also the vehicle manufacturer or anyone involved in modifying a vehicle would also help.