reasoning

Lane splitting is the solution to this. (typical video — in places that force cars to slow down, the mopeds just zip past. Yes, it's not always 100% safe, but if your choice is being stuck for hours in an emergency, or lane-splitting... I'll take lane-splitting any day)

Two-wheeled vehicles also have the advantage of very high gas mileage, which helps with the other major mass-evacuation problem — that gas stations along the evacuation route run out of gas very quickly. (see Hurricane Katrina)

Public transportation might be a viable solution, but it very well may not be, if there is much social chaos, if evacuation is due to disease, or if the grid goes down.

moped requirements

• very good gas mileage (100mpg is typical for decent mopeds)
• two-seater
• have a fuel tank that's upgradable (or, if not, then have a plan for carrying supplemental fuel)
• bonus: light weight (to carry over obstacles (fallen trees, Jersey barriers, other roadblocks), or to store in the vehicle in less-visible places, possibly even up a flight stairs)
  (once people realize that lane-splitting is the only way to leave the city, looters will start looking for two-wheeled vehicles, but will only look for them at ground-level)

refilling fuel

• siphon fuel from vehicles that were abandoned due to congestion
• supplementary fuel tanks
  • detached gas tanks
    • the main requirements are:
      • MUST NOT leak — causes loss of fuel, as well as a major safety risk
      • should be as crash-worthy as possible... at least be able to handle a minor fender-bender
    • suggestions
      • many motorcyclists suggest MSR fuel bottles
      • Fuel bladders are purpose-made for ad-hoc backwoods storage. However, they're usually used for stationary storage. Some people DO use them for hiking. It might be possible to use them for transportation purposes, but having flexible-walled fluid-storage is really bad for stability (motorcycle handling).
      • obviously, hand-portable fuel containers are widely available
    • materials to definitely avoid:
      • glass — it cracks VERY easily, and a crack means spilled gasoline, and spilled gasoline means a HUGE flame risk
      • HDPE plastic — gasoline causes it to degrade (though it can work in a short term situation)
  • attached auxillary gas tanks
    • there are some examples of these: [1] [2]
    • generally, they seem more expensive, and not worth it for a one-off event... they only seem worth it for people who FREQUENTLY need expanded fuel capacity

use beyond urban evacuation