Once you have a root locus, the next question is "How do you use all of the information that you get from the root locus to predict how the closed loop system will respond?"  Remember, you plot the root locus using open loop information about the open loop poles and zeroes, but you are getting information about how the closed loop system behaves.  Here's the story.

• Often, you have some response criteria (response time, allowable overshoot, etc.) that allows you to determine where you want the closed loop poles.
• For a given single closed loop pole location on the root locus, we can calculate the root locus gain that puts a closed loop pole at that position.  For that gain, closed loop poles will be somewhere on the other branches - and you would have to locate them to do a complete prediction of the response.
• Given the value of the root locus gain we can then compute the open loop DC gain.
• From the open loop DC gain we can compute the Steady State response for a step input (the most common test signal for judging system response) and from that we can compute the SSE.
• From the pole locations, we can determine the following characteristics of the closed loop response:
• Frequency of oscillations - if there are any.  The angular frequency of oscillations is the vertical distance of the closed loop pole.
• Time constant (of oscillation decay, if appropriate) from the horizontal distance to the left of the imaginary axis.
• The damping ratio of complex closed loop poles.  In turn, that will allow estimation of overshoot in the step response.