Radical
Tortoise

Mnemonic

A ninja sneaks into the DMV to get a license. Why? Because the tortoise he rides into battle moves so slowly, he needs a special slow-vehicle permit. The ninja slaps the license right onto the tortoise's shell and proudly rides off at 2 mph.
Additional thoughts
Picture a ninja in full black gear sitting atop a giant tortoise shell, a laminated license duct-taped to its back. The absurd slowness contrasts with the ninja's stealth, making it memorable.
Quick recall
A ninja slaps a license onto his slow tortoise mount.