After installing a countermeasure, make sure it is working. If you had a history of accidents
before the change, compare the accident frequency before the change to the frequency
afterwards, and see if it went down. If it did not, look into other countermeasures. You may find
unintended consequences. For example, prohibiting left turns at an intersection may increase the
number of left turn accidents at the next intersection.
If the number of accidents prior to work was small, observing traffic might be more useful than
a before-and-after accident study. If you needed five years of traffic records for the “before”
study, you may need at least five years before any conclusions can be made. After installing your
countermeasure, watch traffic to see if fewer drivers make erratic maneuvers.