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The term “Drag Factor” is not mentioned in engineering mechanics or physics books but it is invaluable when investigating traffic accidents. Drag Factor is the deceleration coefficient for an entire vehicle. The Coefficient of Friction is the deceleration coefficient for a sliding tire. The Drag Factor and Coefficient of Friction are the same, if and only if, all four tires of a motor vehicle are locked and sliding on a level surface.

For years police officers were taught these two terms were the same thing and for the sake of simplicity, we will use the terms interchangeably and say they mean the same thing for the purpose of this discussion. In this article, the drag factor, or coefficient of friction, basically measures the stickiness of a surface such as a roadway, a grassy knoll, a ditch, and the like.

So why does an accident investigator need to know what the drag factor of a roadway surface is and how do they use it? Each surface has its own drag factor, which will determine how quickly a vehicle can come to a stop and how far it will travel while doing so. This answers the question: “How fast were they going?” Which is probably the most common question heard when an accident investigator retains a case.

In an accident investigation, this stickiness of a surface is assigned a number and is denoted with an f. This surface’s stickiness, or drag factors, are determined in a number of ways:

  1. Tables and charts published by accident-investigation authorities and based on thousands of tests. These charts have been validated in court and are used worldwide. They are available from a number of sources, both in print and online.
  2. The use of a drag sled is also common. A drag sled is basically a box that is a known weight; it is pulled at a known force, and the drag factor of the surface is calculated. These devices are be purchased pre-made, or made by the investigator.
  3. A skid test using a similar vehicle, if possible, can be completed. The vehicle drives at a known speed and then completes a hard-braking maneuver to obtain skid marks. These skid marks are measured, and a drag factor is calculated.
  4. Various scientific instruments (for example, the VC-3000) determines the drag factor of a roadway surface. These devices began in the research field and the drag racing world and are purchased for about $1,500.

Knowing the roadway’s coefficient of friction is important as this factor will affect the accuracy of the speed calculation. Let’s take a simple example of a vehicle sliding on the roadway. The vehicle left 4 skid marks that measured an average skid of 200 feet. The investigator presumed the roadway was new asphalt and decided it had a coefficient of friction of 0.8 by using the standard chart of the coefficient of friction for various roadway surfaces.

Using the basic slide to stop formula:

S = speed of the vehicle

d = distance of skid

f = coefficient of friction

The investigator determined the minimum speed was 69.2 MPH. It was later determined the roadway was actually, 10-year-old, heavily traffic-polished Portland cement. This particular cement has a coefficient of friction of around 0.55. Using this coefficient of friction, the minimum speed is calculated at 57.4 MPH. A difference of about 12 MPH from the original calculation.

While additional variable will affect the final calculations, a competent accident-reconstruction investigator must keep in mind that an accurate coefficient of friction/drag factor is important when trying to determine how fast a vehicle was traveling prior to a traffic accident.

by Michael Miranda