Once you unlock the Traffic Coner (by running over 250 traffic cones) you will use this upgrade to shoot traffic cones out of the trunk of your limo. "Aggressively warn" means to hit and kill 12 people with the cones.
Since the cones fly out from the trunk, simply find a large group of people (the beach area, or the airport are the first to come to mind) and press X to shoot the cones at the people. It should only take a minute or two, depending on the groups of people you find and your aim/luck in registering a hit. If you start to shoot and miss, do not move towards the crowd, as they might run - stay stationary and shoot the cones once your upgrade recharges.