what makes those little orange spots on roads?

I see them everywhere; a little bigger than a quarter, rust-colored spots that sort of look like meteors in the ground. How do they form?

Edit: [Here](http://imgur.com/SOy2h1g) is a picture of what I'm talking about.
The orange spots that are seen on roads are called "rail dust". [1] They are caused by the wheels of trains hitting rocks or other debris that is on the tracks. [1] The purpose of the markings on the street is to indicate where pipes, cables, and other structures are located so that they are not hit or destroyed during construction. [2] Beyond that inconvenience, a burst service line can disrupt access to water, power, and other utilities for your home or work. [2] The blister-like bumps, also known as truncated domes and detectible warning pavers, are a part of tactile paving — paving that can be felt. It helps visually impaired pedestrians detect when they are about to leave the sidewalk and enter the street. [3] The bright yellow or red coloring on those sidewalk bumps also helps to alert them that the sidewalk is coming to an end. [3]
The orange spots on roads are most likely due to metallic minerals in the asphalt or concrete reacting with moisture in the air and becoming oxidized [3]. Examples of such metallic minerals include pyrites, which are prominent in certain river gravels [3]. Pyrites can collect and stain the asphalt when the surface becomes wet, and this chemical reaction sometimes causes the spot to swell and balloon a bit before cracking [3]. If the area is not treated, the rust spot can continue to worsen and travel deeper below the surface as it's exposed [3]. Other possible causes of orange spots on roads are brake dust, which can contain metallic minerals, and mineral particles from the atmosphere [1, 2].