What are the blue road reflectors for?

Hey, Matt:

When you're driving down a residential (usually) street, you see the "road Braille," the little raised markers that tell you what lane you are in (or not in, if you cross it), and they are the usual colors of amber, white, red if you're going the wrong way, etc. What are the odd-placed blue markers for? I sometimes notice them at intersections, but then I also notice them at other odd places in the drive space. Please solve this. It's driving me nuts.

-- North Park Driver

There are so many other things on the road to make a person nuts. This seems so mild. Anyway, the next time you see a blue reflector, look at the adjacent sidewalk. You'll find a fire hydrant somewhere nearby. It's a quick way to ID them for speeding fire trucks.