I've thought about putting a left mirror on my handlebar already, and now I'm thinking of buying/building a turn signal setup. With LED's it would be very easy to do, and they could even be made in the shape of an arrow. Would also be rather simple to rig up brake lights based on the front brake only. Has anyone done this? Is it worth it? I ride in traffic regularly, and this has crossed my mind a few times. I was thinking that a few Ni-MH (5 for 6V) of 2.25Ah would be plenty for those lights and for a head lamp too. Just gotta find a good 6V bulb and reflector. Wiring would be simple.

Any input on effectiveness and what not? I just want to make my commuting safer, and geekier.

Back ~20 years ago I had a small tail light to replace the reflector that had the left / right arrows. It had a wire that ran to a controller that mounted on the handlebars. Then all I had to do was slide the control from one side to the other (or the middle) to change how the lights flashed. While it was fun, I'm not sure how effective it was. Drivers don't necessarily look for turn signal on a bike, so they might just wind up looking like a generic flashing light.

It's been my experience that amongst drivers who are the slightest bit responsible and attentive that you get a lot more respect if you abide by the rules, if you bike in a responsible nonintrusive manner (i.e. holding a line and not swerving all over the road when there are no hazards), and if you have equipment such as a traffic vest, bright lights, turn signals, etc that make you seem to be a serious well equiped knowledgeable biker.
Then again there are also drivers who won't see you or respect you no matter what you have.
In traffic, turn signals (or even the use of hand signals) usually gets me a lot more room and respect. For example, if I'm making a left turn and I'm on the right most lane of a 3-lane avenue, as I'm crossing over the lanes in preparation to turn, cars would come dangerously close to me from either the side or back. When I use hand signals after every single lane crossing, that little bit of effort seems to tell the motorist what my intentions are and gives them time to complate what their intentions should be. That usually buys me a hole to move in, I get a spot to ride in almost as big as a space for a car.

Just make sure you have a solid center light for reference. Nashbar also sells a nifty brake light for $10.

This thread covered this recently
https://www.bikeforums.net/showthread...ght=indicators
I've had no problems since using mine of a night, as soon as the indicator comes on you can hear following vehicles slow down.