SPEED TOO FAST FOR ROAD CONDITIONS

As an accident reconstructionist, I’ve heard more wailing about this type of collision and more lame duck excuses than I care to remember. We are living in Canada; it rains and snows here. Roads get slippery when it rains or snows. The aim of the game is avoiding collisions with all kinds of things like poles, traffic signs, telephone and cable boxes, ditches, to name but a few, and last but not least, other motor vehicles.

To do this you must understand your enemy, the slippery road. Just how do we know when a road is slippery? One habit initiated many years ago that has helped me avoid a stupid collision is testing the road shortly after I leave from the house. On your more than likely rather quiet residential road run the car up to 30 km/h and then brake rather hard. You will notice immediately if the road is slippery or not. Anything from heavy dew or a light mist to pouring rain or a blizzard is going to make the road slippery. Some of the more common extremely slippery situations are as follows:

• Rain: It has just begun to rain. It hasn’t rained for several days. The rain begins to mix with the fuels/oils that have been deposited on the road surface. The closer one gets to an intersection the more prominent the oil/water mixture becomes. It is at this place that the vehicles have stopped for traffic lights or stop signs and have time to dump their waste products on the road. The available friction required to slow and stop your vehicle has now been cut in some cases in half. That 10 metres you need to stop suddenly becomes 20 and you don’t have that much anymore. Now imagine this situation on a down-slope and your troubles have just begun. Within an hour of the commencement of the rain this situation should all but disappear; the slipperiest times during a rainfall will be just after the rain has commenced

•”Black Ice”: Temperatures that hover just above the freezing point cause a special problem that is known as “Black Ice”. Black Ice is also known as “glare ice” or “clear ice”. This refers to a thin coating of glazed ice on a roadway surface. It is transparent, which allows the usually-black asphalt roadway to be seen through it, hence the name. It is unusually slick compared to other forms of roadway ice, and as it contains very few bubbles, black ice is very difficult to see. In addition, it often is confused with a wet road, which is identical in appearance. Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway, causing the pavement temperature to drop more rapidly. Black ice can form when the temperature of the road surface is at or below the freezing point whereas the air is normally just above the freezing point. This occurs during dawn periods, or under bridges, near bordering tall trees or anywhere where the shadow of an object blocks the warming of the road surface. Add any amount of water to this surface and voila, instant ice!

There is a notorious location (see Figure Three below) in my city where a combination of several traffic lights, a down-slope and the shadow of the provincial highway overpass create a devil’s mix of black ice covered with oily water especially when the air temperature is just above freezing. For strangers to that location, it’s an instant invitation to disaster. You only have to listen to their descriptions of total loss of control to understand their frustration. • Bridge May Be Slippery: Warning signs are erected for a purpose. Be aware of the potential danger, both under and on bridge decks or overpasses. (Fig. Four) Black ice may form even when the ambient temperature is several degrees above the freezing point of water (0° C) if the air warms suddenly after a prolonged cold spell that leaves the surface of the roadway well below the freezing point temperature.

• Snow:-- Snow at intersections presents another problem. If the temperatures are not extremely cold the constant braking of vehicles especially near stop signs will cause the snow to begin to pack. The more cars that stop there the more slippery it will become as the tires scuffing on the snow produce enough heat to cause some of the snow to melt and then re-freeze almost immediately. Thus the snow begins to turn to ice. With sufficient vehicles stopping there, an ice skid pad is created. Then you, the unsuspecting driver, arrives upon the scene assuming you can stop, as at the last stop sign. Well if someone is still at the stop sign, we have a rear end collision; or if no vehicle is there you go sailing through the intersection into an angled collision with another vehicle, which could even be worse.

• Another common “driving too fast” collision occurs when one inadvertently demonstrates Newton’s First Law of Motion. Sir Isaac determined that if you put something in motion at a constant speed (such as a car) in a straight line then it would keep following that straight line unless acted upon by an external force. Such an external force is found when a car’s tires act upon a roadway. People continue to drive through curves on the road with wet or icy surfaces and expect that their vehicle will go around it as any other time. Well it will not! If you are entering a left hand curve at a too high rate of speed then you should expect to leave the road on the right side and pray you don’t hit anything such as a light standard or telephone pole. The photo below illustrates what can happen if you hit a pole at high speed. Far too many people assume that hitting a snow bank won’t hurt their car; however snow will push against the side of your vehicle when it comes up against it and the soft parts such as the door shells and fenders will buckle. The door frames will not. This can cause thousands of dollars in damage. I’ve actually seen vehicles hit a snow bank that looked as if they were extruded from a lasagne noodle machine. • If you lose it on a right hand curve you can expect to cross into any on-coming traffic. In both instances because the vehicle did not have enough traction (external force) it kept going straight. Now before someone starts complaining that their car did not go in a straight line but rather was spinning around let me explain further. It is the centre of mass of your vehicle that will keep going straight and it is that centre of mass that your vehicle was spinning about as it left the road.

Just a word to the wise here, do not argue with those who have studied physics that a car rounding a left turn will run off the road on the left side because of the “yawing” action (that’s what the above rotation is called). That is not going to happen in this universe; it will leave the roadway on the right. Another issue that makes investigators cringe is when people tell them about their vehicle beginning to yaw as they enter the curve and that they applied their vehicle brakes to attempt to correct it. Well, that is the worst possible thing anyone could do. Any chance you had of correcting your skid flies out the window when one applies the brakes. The car’s tires only have so much traction. When you attempt to steer hard to avoid crossing the centre-line or leaving the road and then apply your brakes you simply defeat the purpose. In those situations, steer without braking! A few words about those forces that I indicated were necessary to allow you to turn corners, to brake to a stop and to accelerate. Understanding this can keep you out of many a traffic mishap.

Every physical system has a limit. Vehicle tires have a limit as to how much traction they can provide for cornering, braking or accelerating. If one causes a vehicle to exceed the traction limit of the tires there will be a loss of control.