Interpreting Crash Attributes

This article provides guidance regarding key crash data fields and crash situations that are important to be aware of when analyzing crash data. It does not include all data fields or all situations, only those that are frequently observed and important for making safety decisions. Crash data uses a finite number of data fields to attempt to describe complex events. It does not provide a perfect nor complete understanding of every situation. The human factors involved in a crash are often unreported or unknown. Crash narratives and diagrams can be extremely helpful when performing a micro analysis of data, but in most situations it isn’t possible to review all narratives.

Crash Data Accuracy

Law enforcement officers have significant responsibilities when investigating a crash. They must secure the crash area, render first aid and other emergency assistance, assess risks related to secondary crashes, determine responsibility, interview witnesses, gather evidence, and input crash information into a formal crash report.

Due to the complexity of these responsibilities and different levels of training officers may have there will always be errors or omissions in crash reports. It is important for crash data analysts to understand this when using crash data. Data should never be ignored or disregarded due to an observed error, analysts should do the best they can with available data and understand that errors are ever-present and cannot be fully addressed or mitigated. Accepting that all data has been through a consistent submittal and review process allows analysts to use data confidently despite errors.

Vehicle Travel Direction

Every vehicle in a crash is assigned a direction of travel. This indicates the general compass direction the vehicle was driving at the time of the incident. This is different from the route direction, the route direction indicates the positive or negative milepost direction and is not the same as travel direction.

Turning Vehicles

It is difficult to interpret the travel direction of turning vehicles in crash reports. The officer should report the direction of the vehicle prior to making the turning maneuver however, this information is often coded incorrectly. For example, when a driver is facing north and turning left to travel west, officers should indicate the travel direction as northbound, but many will select westbound. This makes it impossible to know if the driver was facing west turning left or facing north turning left. When making approach-specific decisions in an intersection analysis it is recommended that the analyst review narratives of at least the high severity crashes (levels 4 & 5, also level 3 when available) to get an understanding of which vehicle movements are most commonly related to severe crash events.

Winding Roads

On roads that curve in different directions we typically assume that the travel direction is the overall primary direction the vehicle was headed. For example, in the image below if a vehicle crashes while driving in the direction of the blue arrow the vehicle’s compass direction would be approximately westbound. However, most officers would code this as eastbound or northbound because that is the primary direction of the road as a whole.

Angled Roads

Angled roads may have multiple options for compass direction, but all officers will generally use one of the same two directions. For example, US-6 between Price and I-70 is oriented in a southeast/northwest direction. So, officers will generally select “south” or “east” for the travel direction of a vehicle headed towards I-70 and “north” or “west” for a vehicle traveling towards Price. There are many similar situations on other Utah roadways.

Manner of Collision

The manner of collision field indicates the general circumstances of how vehicles collided during the crash event. Officers typically select the manner of collision based on where the vehicles are damaged. Grouping crash types only by the manner of collision may limit the crashes of interest. A few examples below help illustrate this issue.

Head-On (front-to-front)

As described in the following section, the “head-on” manner of collision is often misused for left-turn crashes. However, in most crash data conversations and discussions, “head-on” collisions are generally assumed to be the type of crash where one vehicle crosses the centerline or median and collides with another vehicle traveling the opposite direction. This is also very common in wrong way driving incidents. When reviewing head-on crashes make sure to check the location and vehicle maneuvers to determine the type of head-on crash.

Left-Turn Crash Manners of Collision

Crashes where a left-turning and through vehicle collide are coded in a variety of ways. If the vehicles are both damaged on the front, it will typically be coded as a “head-on” manner of collision. If one vehicle is damaged on the side and another on the front, it will often be coded as an “angle” collision. When vehicles are damaged on both driver’s sides of the vehicle they will generally be coded as a “sideswipe opposite direction” collision.  All three of these conditions can occur when a vehicle starts a left turning maneuver and collides with an opposing vehicle. The results are manners of collision that include a mixture of events.

1. “Head-on” crashes will include actual head-on crashes where a vehicle crossed the centerline and left-turn head-on crashes.

2. “Angle” crashes will include both left-turn related crashes and red-light-running crashes involving two through vehicles.

3. During icy conditions, vehicles may rotate before they collide resulting in various combinations of manners of collision that are unexpected.

To avoid confusion, it is important to use the vehicle maneuver and the manner of collision fields together when analyzing crashes at intersections. Any crash that involved a left-turn maneuver should be treated as a left-turn crash, regardless of the manner of collision that was selected. The general preference of the Traffic & Safety Division is that crashes involving a left-turning vehicle and a through vehicle be coded as “angle” collisions.

Single Vehicle Crashes

If two vehicles did not collide during a  crash, the manner of collision is marked “not applicable/single vehicle”. There are generally three types of single vehicle crashes. Separating these types is most easily accomplished using the rollup fields.

1. Roadway departure incidents where a driver left the roadway and crashed on the shoulder, median, or elsewhere off the roadway, often hitting a fixed object, barrier, or rolling over.

2. Active transportation incidents where a driver hit a pedestrian or bicyclist.

3. Animal incidents where a driver hit a domestic or wild animal.

Sequence of Events

The “sequence of events” or just “events” of a crash are described in data using four events per vehicle. Every vehicle has event fields numbered one through four that define what happened to that vehicle in that order. These events allow analysts to understand the “story” of the crash quickly without having to read the narrative. There are several common errors, omissions, and nuances to crash events, some of which are described below.

1. The roadway departure events (listed below) indicate that the driver failed to stay within the lanes of their travel direction. 

   • Crossed Median/Centerline

   • Ran off Road Right

   • Ran off Road Left

Some of these events are caused by poor weather or visibility (ice, snow, rain, etc.), but these events likely include some form of drowsy, distracted, intoxicated, or aggressive driving. However these contributing factors are rarely known or identified because the officer didn’t witness them and drivers are hesitant to admit to them. In some states “lane departure” is used for incidents where a vehicle left their lane but stayed within their direction of travel. In general Utah does not report anything to this level. The manner of collision for such an event would generally be coded as a sideswipe same direction or sideswipe opposite direction.

2. Event codes 40-69 are referred to as the fixed object events. These are used when a vehicle leaves the roadway and hits one of the objects described in these events. The types of objects hit can help the analyst understand and identify potential mitigation measures. Because these objects are never placed intentionally in the roadway, there should always be a roadway departure event prior to the fixed object event. If a roadway departure event is not present, analysts can safely assume that the vehicle left the roadway.

3. A “most harmful event” is assigned per vehicle. This uses the same event codes and is intended to indicate the event that caused the most damage, injury, or harm to that vehicle and its occupants. This does not provide the “story” aspect of the four events but it does help identify primary injury-causing events. A “first harmful event” is assigned per crash. This also uses the same event codes and is intended to identify the first event that caused injury to a person or damage to a vehicle. In most cases when querying data for a specific event analysts should use all four sequence events, the most harmful event, and the first harmful event.