Highway engineers put water barrels at highway and tollway exits in case a car goes out of control and, instead of hitting the cement bridge pillar, hits the barrels. They place them at the exit ramps and not the entrance ramps because the speeds that the cars are traveling at exit ramps are greater than those at entrance ramps.
Water acts to cushion a car's impact. These water-filled barrels are much more forgiving on a car than solid objects. They also give weight to the barrels (this allows the engineers to not have to secure the barrels to the ground). When the barrels need to be replaced, workers simply drain the barrels of the water so it is easier to move them. The barrels are colored in caution colors (orange and red) as well as reflective colors (yellow and light green). There are also reflective strips put around the barrels to make them more visible at night.
Water barrels are placed in a diamond formation (with the point in front) to reduce the speed on impact. Even if the vehicle does not hit the barrels head-on, they are strategically placed so the car will always hit at least several barrels. They are not secured to the ground, because if they were, they would not give when they are hit.
