Yup, it refers to the “redshift” phenomenon.
Yup. When two objects are moving towards one another at relativistic speeds - at asignificant fraction of lightspeed - the light from one object hitting the other object will be more blue, because the wavelengths of light are closer together. Redshift - the wavelengths are farther apart - happens when objects are moving further away. This phenomenon is an important piece of evidence that the universe is expanding.
These effects are directly analogous to the doppler effect heard when an ambulance goes past with the siren going. The bumper sticker implies that the person overtaking is approaching our car at relativistic speeds, certainly too fast for city driving.
These are from NancyButtons.com, but they’d make good bumper stickers.
Don’t get even–get odd!
The Two Rules: 1) Don’t tell people everything you know
Buying a book is buying the illusion you will have time to read it
Money can’t buy happiness. You have to turn it into books.
I prefer to remain anomalous
There is no prostitute for careful editing of your own work
pointing at my previous post Er, yeah, it’s blue shift - opposite to redshift. 
I hear about redshift more so my brain went there right away.
A car I used to have had Stupid People Shouldn’t Breed on its rear bumper.
Man, I loved that sticker. People smart enough to “get it” would nod and laugh. People who didn’t get it, well…
I used to have a “Republicans for Voldemort” bumper sticker. Even my Republican parents thought it was amusing.
A lot of good suggestions from y’all, my favorites so far being the ebonics one and the binary one (did a spittake on that).
Favorite that I’ve ever seen on another car:
Honk if You are Elvis
Favorite I’ve ever had on my own car (from my deadhead days):
Who are the Grateful Dead and why do they keep following me?
Favorite that I’ve actually only seen on a T-shirt:
Jesus Loves You (but I’m His favorite)
I’ve always wanted one that says “Sheik Djibouti”
Then there’s the great “My karma ran over your dogma”.
“In the event of the Rapture, can I have your car?” (May need explanation: you see “In the event of the Rapture this car will have no driver” bumper stickers here sometimes)
“If man wasn’t meant to eat animals then why did God make them out of meat?”
One that I would never put on my car but damned near caused me to have a wreck from laughing so hard when I saw it:
I BEAT CANCER!!!
and by “cancer” I mean my wife
I may have to put this above* my other geeky favorite:
Reunite Gondwanaland!
I have no idea why I find this so funny. I’m not really into geology. But for some reason it cracks me up every time.
*When I get a car, that is.
True… and yet, nobody ever thinks of himself or herself as “mean.” And I’ve known several true pricks who have that very bumper sticker on their car.
See, that’s what happens when people assume “mean people” is synonymous with “Christians” or “Conservatives”. “Mean” is a state of mind that’s quite compatible with ANY belief (or nonbelief) system.
Conserve Energy – Commute with a Hamiltonian
(Hilarious quantum physics joke/pun)
(PS, I didn’t think it up, it won an American Physics Society contest. I wish someone would print some.)
Honk if you love honking.
To Long?:
Become a Ringwraith
First 9 applicants receive a free ring.
“中文系: We Learn Chinese So You Don’t Have To”
I got that one from the Defense Language Institute (where I flunked out because I couldn’t learn Chinese. Ironic, yes? I have it stuck on my computer case).
What makes this sticker really funny is that, for some reason nobody could figure out, the last character (系) is done wrong.
Ok, but won’t this attract the attention of Sauron?
My computer had “America: Love it or Leave it”, which, as an American immigrant to Canada, has always cracked me up.
That that on my old laptop, along with one that said “Intelligent Design” deliberately misspelled in Comic Sans. My current one has a bunch of Obama stickers on it.
What can I say? I don’t drive!
Part of me wants to get that bumper sticker, deploy overseas, and slap it on a Humvee on the other side of the world.
Of course, I’ve got a friend who makes vinyl decals, including one designed to go on tailgates that reads “Zombie Outbreak Response Vehicle” which I was sorely tempted to slap on the back of one of the Security Forces jeeps on base here.
I’m sure they’d appreciate it, but it still strikes me as poorly considered XD
This is excellent. I may have to actually do this one. In particular because it would be perfect on our black econobox.