- Busses. How do you make them half width? (or perhaps, uncharitably, do you think public transportation is worthless?). If busses stay the same width how do you write the driving regulations for mixed width vehicles and retrain the driving public so that fatalities do not skyrocket.
- Bulk delivery vehicles. Same as above.
- Half width means less vehicle stability at speed. Reducing vehicle height is constrained by human size. Worse stability means reduced speeds to reduce fatalities. Consider San Francisco, where a large proportion of the city cars are from commuters traveling 30-60 miles per day. What are the rules for commuter roads with mixed width vehicles?
- Reduced width would likely mean smaller wheels, increasing rotational speed. Tire and brake pad wear is now a non-trivial factor of car pollution. Half width vehicles would / could make this worse.