Tread Lightly

Tread Lightly

Motor vehicles are essential to all aspects of society, including the criminal aspects. Most crimes involve a motor vehicle and often, the only evidence left at a crime scene is the tire track. Therefore, investigators carefully search crime scenes for clues left by vehicles. A tire impression is the product of direct physical contact of the tire on a surface resulting in the transfer and retention of characteristics of that tire onto the surface. The investigator needs to be able to discern the position and direction of the impression, the tire track width, tread width and other information that would aid in the reconstruction of the event.

During the summer months of 1998, a string of murders occurred in the muddy swamp lands of Southern Florida. After thoroughly combing the scene for evidence, investigators at each crime site found only a set of tire impressions leading in and out of the site and a WalMart smiley sticker glued to each victims’ nose. After months of searching, investigators were able to narrow down their investigation to three WalMart employees that worked and lived near the area where the victims were found.

In an effort to minimize additional wear and alteration to the tires in question, the vehicles of all three suspects were immediately collected. Once the vehicles arrived at the lab, investigators made test impressions with the tires still mounted to the vehicle frame. A test impression of each vehicle’s tires was made by inking the tires and then rolling the vehicle forward on a clean surface.

Once the test impressions were made, investigators of this case then needed to compare the test impression of each suspect’s vehicle to the impression left at the crime scene. They first measured the tire track width for each car. The tire track width is the distance between the center of the tire mounted on one side of the vehicle to the center of the tire mounted on the opposite side of the vehicle. For more accurate results, analysts measured the distance between the outside edge of the left tire impression and the inside edge of the right tire impression.

Two of the three suspects’ vehicles matched the tire track width of that found at the crime scene, so additional measurements were necessary: the front and rear tire tread width. The tire tread is the pattern of the tread design on the tire. To accomplish this task, investigators measured the distance across the tread of a tire in the front of the vehicle and then a separate measurement of the tread width of a tire on the rear of the vehicle. Because the rear tire will have run over the front tire track when the vehicle is proceeding forward, it is not possible to distinguish between the front and rear tire tread width for the crime scene impression.

Finally, investigators were able to narrow down their search to one main suspect. However, in order to make a positive match between the suspect’s vehicle and the crime scene vehicle, investigators identified individual characteristics of the suspect’s tires such as nicks, cuts, wear patterns, pebbles, etc. Based on the comparison of the tire impression evidence left at the crime scene to that of the suspect vehicle, 34 year only Michael Porter, a two year WalMart employee of Tampa Bay, was convicted on all charges and sentenced to life in prison.

Name: ______Date: ______Pd: ______PG: _____

Trackin’ On

Background Information:

Someone has run over Sheriff Hosey’s mailbox for the third time. Even though he is a patient man, the sheriff decides to find out who keeps destroying his mailbox. This third incident occurred at about 2:00am Saturday. As soon as he heard the familiar crash, the sheriff ran to his front porch to get a glimpse of the offending car and its license plate. Unfortunately, the car was moving too fast for him to read the license plate number.

The sheriff saw enough of the car, however, to narrow down his list of suspects. On the rear bumper was the familiar sticker “DHS Lions.” In his yard, on both sides of his crushed mailbox, are a distinctive set of tire prints. On Monday morning, Sheriff Hosey stood in the DHS parking lot and made a list of three vehicles with the DHS sticker. He had the cars impounded to be taken to the lab at his police station.

Your Mission:

Your mission as the lead CSI team in Timonium is to compare and contrast tire impressions left on Sheriff Hosey’s yard to impressions made by the three suspect vehicles. You do not want to let your boss down so you need to make sure your procedure for this assignment is going to ensure you an arrest.

Procedure: Follow the sequence chain for each suspect vehicle.

Data: Use the table below to record your data or create your own.

Set Letter: _____
Example
Crime Scene Impression
Suspect Vehicle 1
Suspect Vehicle 2
Suspect Vehicle 3

Post Lab Questions: Answer in complete sentences.

1.  Which suspect ran over Sheriff Hosey’s mailbox? Explain how you know.

2.  Explain why is it important to analyze data from both the front and the rear tires of a suspect vehicle.

3.  Tread patterns are often very distinctive. Could you have identified the car belonging to the mailbox vandal by tread patterns?

Analysis Questions: Answer in complete sentences.

1.  What was the most important piece of data that you collected from the tire impressions? Why?

2.  In the future, how would you amend your procedure to get more accurate/complete results?

3.  How valuable to do you think tire impressions are as evidence in a murder trial?