What can be determined from a motorcycle’s skid mark and the roadway evidence it left behind? The obvious, that the motorcycle crashed; but what more? From a recent case, starting with the pre-impact skid mark, a lot was determined about the driver’s (and passenger’s) input into the crash. A single skid mark started at the beginning of the collision location, a common piece of evidence at a motorcycle crash. Taking that mark and analyzing it, several things were determined.
It started out light, got dark, then got light again. The single skid mark was over 100 feet long. The first interpretation is that the rear brake was locked up, a simple observation that is very important. This is an indication that the motorcycle was not achieving 100% of its braking capability. Studies show that just using the rear brake of a motorcycle yields an average of 40% of the vehicle’s braking (40% of the available braking comes from the rear wheel while 60% comes from the front wheel). This alone does not mean that the driver did not apply any front brake, but it shows that at a minimum 40 % of the available braking was in use. 60% of the brakes (front brake) may not have been used or was used inefficiently.
Further inspection showed no or little front brake had been applied at the beginning of the skid. Lack of front brake application or improper front braking is not unusual. According to studies and observations, novice or inexperienced riders often max out the rear braking and avoid, or do not effectively use, the front brakes. Notice in the photograph how the rear wheel skid is not straight but does a “Lazy S” pattern. This is because the rear wheel was drifting to the right of the front wheel’s path as the front wheel continued straight in its travel. This drift was not wide, and the rear wheel started to realign behind the front towards the end of its skid. As the motorcycle started to straighten back up, the driver and the passenger of the vehicle started to slide forward on their seats. The driver pushed up onto the tank, the passenger onto the back of the driver, and the weight of the motorcycle shifted forward compressing the front suspension. This is shown by the rear wheel skid getting thinner and lighter as it began to lift off the road. Near the end of the rear wheel skid, the driver locked the front wheel brake. The investigating officer may not have noticed a short front wheel skid mark, which is common. It may not have been thick or dark or it may not have left a recognizable mark, but when the front wheel of the motorcycle locked, the vehicle went down. Think back to your days riding a bicycle, when you over applied your front brake, bad things happened. Typically, you went over the handlebars and onto the ground. Some motorcycles will lift in a similar manner, but in this situation the rear wheel was still a little to the right of the front wheel’s track and the motorcycle tossed forward onto its right side (which matches with the physical evidence on the motorcycle also).
When a motorcycle spills, its handlebars, footrests, and different portions of the motorcycle contact the ground and begin to leave evidence. In this case, gouges and scratches were present on the road’s surface just beyond the end of the skid mark. This showed that the motorcycle was on its side. The gouges and scratches started out flowing in the same direction that the motorcycle was originally traveling then skidding. This shows that (aside from striking the asphalt) the motorcycle had not yet collided with anything. It continued to slide on its side for a distance before being redirected hard to the right. The redirection occurred because the motorcycle had impact with another vehicle. Damage on the motorcycle matched up with the contact with the opposing vehicle. The location of the redirection aligns with the travel path of the other vehicle. After impact with the other vehicle, the motorcycle continued to slide on its side, rolled to its left side, and came to a rest. Other types of physical evidence may be mixed with the scratches and gouges such as paint, rubber, plastic, clothing fiber, or even tissue transfer, but these types of evidence are generally short-lived.
All the information gleaned from the physical evidence of this motorcycle’s crash was useful in calculating a speed for the motorcycle as the driver reacted to the perceived threat and approached the crash. From what was determined by the skid mark, the scratches, and the gouges, braking efficiency was considered and coefficient of friction values were ranged. Reading the roadway evidence not only painted a picture of what the motorcycle did prior to and beyond the crash, but it supported the speed range that was calculated.
Aaron (Al) Duncan II, ACTAR, is a senior vehicle reconstructionist with Warren. Prior to joining Warren, he worked for 23 years as a South Carolina Highway Patrol Trooper to include 10 years as a Multi-Disciplinary Accident Investigation Team (M.A.I.T.) member. Al has investigated in excess of 1000 vehicle accidents and incidents, as a trooper. Al has testified multiple times in state courts and has been a court qualified as an expert in accident investigation and collision reconstruction. Al’s work expertise focuses on investigating and reconstructing vehicle collisions involving single and multi-vehicles, animals, pedestrians, motorcycles, heavy trucks and commercial vehicles. He is also a skilled user of forensic mapping technology and computerized collision diagramming software for collision scene analysis. Al is experienced in the data download and analysis of airbag black boxes (Crash Data Retrieval Units) in automobiles, pickup trucks and SUVs. He holds a Bachelor of Science Degree in Political Science from Lander University in Greenwood, South Carolina and completed the Law Enforcement Basic Program at the South Carolina Criminal Justice Academy in Columbia, South Carolina.