Walking around your city, you might have noticed spray paint markings on the street and sidewalk. Clearly they’re there to mark something, but what? Turns out, construction workers aren’t just doodling for fun—those marks are there to protect you.
The main purpose for markings in the street or on the sidewalk is to indicate where pipes, ducts, cables, and other structures are located, and what direction they flow in, so that they’re not hit or destroyed during construction. If you’ve ever seen a water (or worse, sewage) pipe get busted, you know how bad it can be for you and everyone else trying to pass through the area (and of course, it’s awful for the construction workers). Beyond that inconvenience, a burst service line can disrupt access to water, power, and other utilities for your home or work.
In addition to helping construction workers keep underground structures intact, your city might also use spray paint markers to track services performed, like the ones San Francisco uses to confirm that a sewer has been treated with methoprene (which combats mosquitos breeding in the drain water).
Update 5/5/2017, 10:53 AM: In response to our astute commenters, we’ve added information about the 811 program and what marking colors mean.
Construction workers and city services aren’t the only ones who use these markings — you might have asked your city to mark pipes and cables on your property before digging up a tree stump or making other changes. This service is known as 811 and the American Public Works Association has worked with them to decide what certain colors mean. Here are the ones for my local 811 service (yours are probably similar).