We aren't talking conventional vehicles with only one person in them - that's wasted space and mass. We mean purpose-built one person vehicles.

Actually, they already exist in mass-produced, fuel-efficient form. They're called motorcycles, mopeds, and motor scooters. They suffer from higher accident rates and more severe accident injuries then large cars, but in many parts of the world they dominate the vehicle fleet.

You also get wet when it rains...

SOme modern vehicles like the european three wheeler discussed above are trying to bridge these obstacles with a safety cage and weather protection. They may be interesting in the future but will remain a niche/leisure market until the price of fuel balloons again, imho.