Narrow vehicles fit 2 side by side in a lane big enough for one car or truck. Obviously not everyone will switch to narrow-vehicles at once, but the more narrow-vehicles on the road, the less congestion occurs with the same amount of commuters.
Narrower trucks are possible for some things but wide and large trucks will continue to be necessary for larger items and loads.
LLIT, there are virtually no narrow vehicles on the road that would support two of them running side by side in the width of one lane.
Thus why would one make these changes when there is NO DEMAND for them?
Even if you made them, and restricted their use to narrow vehicles to encourage the adoption of narrow vehicles, the immediate effect would be to take a lane out of service.
And it would stay out of service for a LONG time. (like forever)
Good going ACE.
As to the vehicles, lane widths range from 10 ft to 12 ft, but even if we only limited your change to 12 ft with lanes, allowing a minimum of 1.5 ft on either side of the vehicle, that limits the width of your narrow vehicle to 3 ft.
Or essentially about the width of a motorcycle.
Got any prototypes for an enclosed vehicle that is that narrow?
Because you would be mixing these lightweight vehicles on the road with all the heavy trucks and SUVs that are already in service.
And NO, a truck this width will not be useful for anything.
That's just one part of the solution anyway. Logistical changes in the proximity of home, work, and shopping are necessary to reduce the average distance and frequency of motorized travel.
Your problem is you are somehow convinced that there is a shortage of ENERGY.
There is no shortage of energy on the planet, even if we were to phase out 80% of fossil fuel use over this century.
When you add up the energy available to us via direct Solar, Wind, Geothermal, Nuclear, Biofuels and Hydro and then supplant it with about 20% fossil fuel use where the alternatives are not viable (Ships, aviation fuel or back up generators at hospitals might be examples) there is no shortage of energy.
So far all your solutions are based on curtailing modern life to go back to roughly an 1800s style existence.
Good luck recruiting followers on your noble path to self sufficiency.