I'm planning on making one, but I'm going to use a microcontroller. Actually cheaper than a 555, zero parts except for the chip itself and the driver transistors, and I can get complex behaviors.

I think a single blinking light is going to be useless. If you're turning you want the OPPOSITE light to come on solid and the turn light blinking. The problem is if you just have one light blinking, and the lights are only a few inches apart, it's not clear what is going on.

Also all other blinkies on the bike should STOP blinking otherwise they just confuse things.

So my plan is to have a pair of lights in the back flashing with something like the Planet Bike Superflash pattern (with 3 or 4 quick strobes and then wait about .7 seconds) normally. When I put on a left turn, the right light goes solid on and the left one goes into a standard blink mode, .25 seconds on .25 seconds off for 2 flashes per second.

I'd also have front flashers, and either an auto-off delay or a beeper that told me the flashers had been on for > 15 seconds.

All that is still just one single 8 pin chip, no external parts other than the switches and the driver transistors.