Brando's Traffic Prompters are cheap at US$16 per light and are meant to promote highway safety. You can even combine a red one with a blue atop your car to emphasise your state of emergency. Just don't be surprised when you're busted for impersonating a police officer.
The powerful magnets at the base of these presumably all-weather LED strobes can hold fast to "any metal surface," while D-cell batteries mean you don't have to plug them into your lighter jack. Still, even though the Prompters strobe rather than swirl, I'd be surprised if they weren't illegal in all 50 states.
When I turned to gadgets.brando.co.hk for a little guidance on this conundrum, all I got was, "Keep pedestrains or peoples on the surrounding of the Lights." [sic]Thanks, Brando—thanks a mil. [Brando via Red Ferret]