Documenting the collision scene: evidence, photographs and measurements

When Kowalczyk took over the Kingsway investigation at 02:15, he walked the scene twice before he touched anything. The first time he walked from east to west along the pickup's path, starting well beyond the first visible mark, looking at the road with his flashlight held low so that the beam would raise the texture of any marks. The second time he walked the hatchback's path, from the eastbound lanes through its turn to its final rest against the curb on the north-west corner. Only then did he decide what to photograph, what to measure and in what order. 'The scene tells you a story,' he tells the class. 'Your job in the first hour is to write it down before anyone changes it.'

This lesson covers the kinds of physical evidence found at collision scenes, how they are photographed and measured, and how investigators turn measurements into a scale diagram. The aim is a record so complete that another investigator, or a court, could understand the scene years later without ever having stood in the intersection.

Marks on the road

Tire marks are the most important physical evidence at many collisions, because they show where vehicles went and, sometimes, how fast. A skid mark is made by a tire that is locked and sliding without rotating; it is usually dark, straight and of even width, and it begins abruptly when the wheels lock. Vehicles with anti-lock brakes, which now includes almost every car and light truck on the road, rarely leave classic skid marks, because the system releases and reapplies the brakes many times a second. Instead they leave lighter, sometimes broken marks, often called anti-lock or ABS marks, which can be hard to see and easy to miss.

A yaw mark, sometimes called a critical speed scuff, is made by a tire that is rotating but also sliding sideways, as happens when a vehicle is steered too sharply for its speed. Yaw marks are curved, and they often show diagonal striations across their width. Lesson 11 explains how their curve can be used to estimate speed. A scuff mark is a short, irregular mark made at impact, when a tire is suddenly pushed sideways. Investigators also look for gouges and scrapes in the road surface made by metal parts dragged or driven into the pavement at impact; these are often the best evidence of the exact point of impact.

On Kingsway, the pickup's front tires had left two light anti-lock marks in the westbound curb lane, one under each front wheel. As Kowalczyk finally measured them, the marks began about 79 feet east of a short, sharp scuff and a set of gouges in the asphalt, where the pickup's front bumper and frame had been driven down by the impact. The first officers had seen the marks start about 5 feet closer to the gouges; with his flashlight held low, Kowalczyk found the faint true start of the left front mark, which was why he extended the perimeter. Beyond the gouges, a curved line of fresh scrapes led north-west to the hatchback's final rest, and a scatter of broken plastic and glass covered an area about 30 feet by 20 around and beyond the gouges.

Marks on the vehicles

The vehicles themselves are evidence. Investigators photograph and measure the damage to each vehicle, a process called crush measurement, which can later be used to estimate the energy absorbed in the collision. They note the direction of the damage, transfers of paint between vehicles, the positions of the gear selector, light switches and seats, whether seat belts were worn, airbag deployment, and the condition of the tires and brakes. They also note anything that may have distracted a driver, such as a phone, food or an open drink, and where it was found. On Kingsway, Kowalczyk noted that the hatchback's right side was pushed in a maximum of 19 inches at the front passenger door, that both front airbags and the right-side curtain airbag had deployed, and that both occupants had been belted. In the pickup, a phone lay on the floor in front of the passenger seat, with its screen cracked.

Photographing the scene

Photographs are the most efficient way to record a scene, but only if they are taken systematically. Investigators work from overall photographs, which show the whole scene and its approaches from each direction, to mid-range photographs, which show the relationship between items of evidence, to close-up photographs of individual items, taken first without and then with a scale or marker in the frame. Every photograph is recorded in a photo log with its number, time, location, direction and subject. Night photography needs more care: investigators use a tripod and long exposures or powerful lighting, and they photograph the same marks again in daylight if the scene is still available.

Kowalczyk's team took 212 photographs on Kingsway that night, in addition to the 14 Reyes took before the rescue. They also took a series of driver's-view photographs from a vehicle like the pickup, parked at points along its path with the camera at the driver's eye height, and from a vehicle like the hatchback in the left-turn position. Those photographs became important in lesson 11, when the investigators had to decide what each driver could have seen and when.

Measuring the scene

Photographs show what evidence looks like; measurements show exactly where it is. The basic principle is to locate every item of evidence in relation to fixed reference points that will still be there after the scene is cleared, such as a curb corner, a hydrant or a utility pole. Two traditional methods are still taught because they need only a tape measure and can be checked in court: the coordinate method and triangulation.

In the coordinate method, the investigator picks a reference point and a baseline, usually the edge of the road or the curb line. Each item is located by two measurements: its distance along the baseline from the reference point, and its distance at right angles from the baseline. In triangulation, each item is located by its distances from two reference points, which forms a triangle that can be redrawn later. Triangulation works better when there is no straight edge to use as a baseline, such as in a parking lot or on a curve.

Total stations, scanners and drones

Many services now measure scenes with a total station, a surveying instrument that measures angles and distances electronically to a reflector held over each item, and records thousands of points accurately in an hour. Others use three-dimensional laser scanners, which record millions of points in a few minutes, or drones that take overlapping photographs from above so that software can build an accurate map of the scene, a method called photogrammetry. These tools are faster and more accurate than tape measures, but the principle is the same: every point is fixed in relation to permanent references, so the scene can be rebuilt later. Kowalczyk used the unit's total station on Kingsway; the drone was not flown because of the darkness and a light rain that began at 03:30.

The rain mattered. Wet asphalt has a lower drag factor than dry asphalt, and the collision happened on a dry road. Kowalczyk recorded the time the rain began and made sure that the measurements of the marks and the photographs of the road surface were completed before then. He also noted the air temperature, the road surface type and its condition, because lesson 10 shows how the drag factor depends on them.

The scale diagram

From the measurements, investigators draw a scale diagram of the scene, showing the roads, lanes, signals and signs, the marks, debris, gouges, final rest positions and the reference points. Diagrams drawn at the scene are field sketches, not to scale but fully labeled with every measurement. The final diagram is drawn to scale, today usually with computer software, and becomes an exhibit at trial. The scale must be stated on the diagram, along with a north arrow and a legend.

Electronic and other evidence

Modern collisions leave electronic evidence as well as physical marks. Most vehicles now have an event data recorder, often part of the airbag control module, which records a few seconds of data before and during a crash, such as speed, throttle, braking, seat belt use and steering. Phones record calls, messages and app use with exact times. Dashboard cameras, traffic cameras and nearby businesses' security cameras may have recorded the collision itself. Signal controllers record timing, as lesson 8 described. Each source needs its own legal authority to obtain, which lesson 11 covers, and each must be secured quickly before it is overwritten.

On Kingsway, the electronic evidence included Siddiqui's dashboard camera, which had recorded the intersection from the Mill Street stop line; the city's signal log; a bank security camera that recorded part of the westbound lanes; the event data recorders in both vehicles; and Halloran's phone. Kowalczyk asked Siddiqui for permission to copy his camera's memory card at the scene, which Siddiqui gave in writing, and he had an officer go to the bank's security company first thing in the morning.