Click here to download the catalog as a PDF file. To view this site you need Adobe Flash Player and your browser must allow javaScripts. Go here to get the latest Flash Player. CARS AND CRASHES SURvIvING IN yOUR MOBIlE OffICE. SUZI HUNTINGTON Roadway R oadways are chock full of good stuff to read — stop, right turn only, ped xing — to name just a few. We also read the lane markings — double yellow lines, simulated islands, two-way left turn lanes, mandatory exit lanes, blah, blah, blah. But what about skid marks?You know, those black marks left in the road — how do we read those? Skid marks or tire scuff can give us quite a bit of useful information, like how fast a car was going, what brakes were working, braking skid, versus acceleration skid but you have to know what you’re looking at. Here then, is a quick and simple lesson in skid analysis. For you traffic geeks, this is basic stuff, so no hate mail please. Reading The This skid shows how the wheels lock-up independently. Its's the shorter of the two skids and shouldn't be used for your speed calculations. ID t here are several types of skid or scuff marks commonly seen at most collision scenes, the most common being straight, locked-wheel braking skid. These marks are usually made when a driver, sensing something is horribly wrong, mashes on the brake pedal. One or more of the brakes will lock up causing the tire to stop rotating, dragging the locked wheel across the pavement and the rubber is then scuffed off onto the roadway. The end result is that nice, straight, uniform black mark. If that mark makes a sudden change in direction it means the driver struck or was struck by something. A critical element of identifying this kind of skid is to determine which tire left the mark. More importantly — was it a front or rear wheel skid. There’s a difference? Yup, and here’s how you can tell them apart. Front wheel skids will be slightly broader than rear wheel skid and they’ll be darker on the outer edges of the mark. This is because of the weight shift during the sudden deceleration, causing the front end of the car to dip and all that weight is transferred onto the front tires. The tire flattens out a little, but the middle of the tread surface cups inward thus making the outer edge of the mark darker. The obverse is true for rear wheel skid — the weight shift lifts the rear of the car, taking weight off the back tires and therefore less tire surface is in contact with the road. So rear wheel skids are slightly skinnier and will have a more consistent tread mark. Once you’ve identified these marks, what do you need to do? First, you should photograph them and then measure them. Remember that inexpensive tape measure I told you to get — now’s the time to use it. You should measure all the skid marks and document them in your collision report, but you’ll only use the longest skid to calculate an approximate minimum speed of the vehicle. What’s that — approximate, minimum speed? You should never use absolute figures when determining speeds because some expert rodeo clown witness will do much more in-depth computations and be able to come up with a slightly different number than you. And if you report an absolute number, that rodeo clown will make you look like a bigger clown than he is in court. So just don’t do it. And another thing, you’re only giving a minimum speed — it could be faster. For IdentIFIcatIon PurPoses Note the dark, outer edges of this skid mark, it's a tell-tale sign of front, lockedwheel skid Pie-RSquared ow that you’ve got your measurements you can plug them into the following algebraic formula to calculate a minimum speed. For some of you math wasn’t your strong suit in high school and it may have been a while since you used algebra, but this is easy stuff — so stay with me. You’ll need N 30 WWW.AMERICANCOPMAGAZINE.COM • SEPTEMBER/OCTOBER 2009