All of us can save a life -- or maybe multiple lives -- by reporting suspected drunk drivers. If you see someone driving erratically and dangerously, you want to stay a safe distance away from them. However, if you can describe the vehicle and its location to a 911 operator, police may be able to get to the driver and pull them over before they hurt someone, including themselves.
As soon as you can safely make the call to 911 (or one of your passengers can), immediately tell the operator that you're reporting a suspected drunk driver. Give as many details as you can about the vehicle, such as color, make and model. If you can get all or even part of the license plate number, that's even better.
Give the precise location, such as the nearest highway exit or cross street. Report which direction the vehicle is going.
Tell the operator precisely what the driver is doing that's unsafe. Are they straddling lanes, ignoring lights or signs, slowing down or stopping for no reason or even driving in the wrong direction on the road?
Even if the driver isn't drunk, they may be distracted, drowsy or even in the midst of a medical emergency. You can't get in trouble for reasonably suspecting that someone is drunk based on their dangerous driving. Whether they're under the influence is up to the police to determine. You also aren't required to give your name when you make the call.
As noted, keep a safe distance from the driver. Don't try to follow right behind or alongside them or take the law into your own hands. If you aren't able to avoid the driver, and you end up in a crash, it's wise to explore your legal options for seeking the compensation you need to deal with medical bills, damage to your vehicle and pain and suffering.