Which skid should be used to figure the estimated speed of a vehicle using the formula square root of 30(d)(f)?

To estimate the speed of a vehicle involved in a skid using the formula square root of 30(d)(f), it is essential to consider the length of the skid marks left by the vehicle. The longest skid is the most relevant because it typically represents the maximum speed at which the vehicle was traveling before the driver initiated braking. Since the longest skid mark indicates the distance over which the vehicle came to a stop, it provides a more accurate estimate of the speed compared to shorter skids.

The formula essentially calculates the speed based on the friction involved during the skid—of which the longest skid mark gives a better representation of the braking process and dynamics at play. This value is crucial because it accounts for variables such as road conditions, tire rubber, and braking strength, all of which are cumulative in influencing the longest skid distance. Therefore, utilizing the longest skid mark ensures that officers can derive a more precise estimate of the vehicle's speed at the time of the incident.