Going strictly by the OP's question, I recommend the Monoprice 8320. Noise isolating, in-ear headphones for less than $9.00 shipped. That's right, NINE DOLLARS all-in for audiophile quality sound. Check out the review here.

I won't use them for biking, but they do block out most traffic noise while out for a walk. And they sound terrific.

If you want hear traffic, you could leave the right earbud in and not use your left earbud so you can still be aware of your (most) of your surroundings.