Posted Speed, Travel Speed, Impact Speed and Change-in-Speed – Do You Know The Difference?

There is no shortage of internet crazies with opinions on everything and a long list of brain-washed followers willing to swallow whatever is put before them. The analysis and reconstruction of collisions is no exception. While it is important that members of the public express their opinions and concerns about the collisions reported to them, there must also be a willingness to accept that those opinions may not be fully informed. As such the following is a brief discussion of one aspect of collision analysis dealing with vehicle speed. Knowing the difference between various descriptors of speed is essential if anyone wishes to provide an informed opinion on collision and injury causes.

For example, Posted Speed is the maximum speed seen posted on signage along any roadway. Often this Posted Speed is about 10 km/h lower than the “Design Speed” of the roadway. In the Province of Ontario many urban roads contain a Posted Speed of 50 km/h. On rural highways 80 km/h Posted Speeds are common, and on high speed expressways a Posted Speed of 100 km/h is common. The internet is full of “expert” opinions that road safety is dependent on the specific speed that is posted on roadside signage. Thus, if a Posted Speed of 50km/h is reduced to 40 km/h this will result in a dramatic safety benefit. Presumably the belief is that drivers are religious followers of Posted Speeds and when these speeds are reduced the speeds travelled on a road segment will also be quickly reduced. There is a naïve belief that the reduction of injuries and deaths are greatly correlated to lowering Posted Speed.

Travel (Operating) Speed is another descriptor which describes the speed at which vehicles travel on a road segment. A specific type of travel speed is the “85th Percentile Speed”, or the speed at which 85 percent of vehicle are observed to travel, at or below that speed. The 85th Percentile Speed is often used by roadway personnel to decide whether Posted Speeds are reasonably set.

In my numerous documentations of vehicle motions using multiple video cameras, I am able to obtain a very detailed assessment of the precise Travel Speed of each vehicle unit where my testing is conducted. While some variations exist, average Travel Speeds are often 10 km/h higher than the Posted Speed.  I have also observed that a good 20 percent of vehicles possess a Travel Speed that is 20 km/h or higher above the Posted Speed. The greatest difference between Posted Speed and Travel Speed that I have documented is on the 400-Series expressways of Ontario where, in some locations and at certain times, Travel Speeds of 130 km/h are not uncommon.

There are common postings on various social media sites that Travel Speed is directly proportional to the severity of injury and probability of death. The commonly held view is that a vehicle travelling at a certain speed will result in a certain severity of injury or a certain probability of death. These generally accepted points of view never provide the specific basis for these conclusions. Whatever research has been conducted the travel speed of a vehicle would be a difficult parameter to determine unless it was obtained from event data recorders. And it has never been revealed which research has specifically used this source as the basis for determining Travel Speed.

A more reliable parameter in terms of predicting injury causation and probability of death has been the Impact Speed of a vehicle. Travel Speed and Impact Speed are not the same. A vehicle may be travelling at 80 km/h and, without any interventions, may possess an impact speed of 80 km/h. Yet in another instance of a vehicle travelling at 80 km/h the driver may apply his brakes reducing the vehicle’s speed to 40 km/h at impact. Those two scenarios are vastly different in terms of injury/death potential. Meanwhile the concept of “velocity” is put into play when we consider the directionality of that impact speed. Two vehicles coming into a full-over-lap head-on collision at 80 km/h have the potential of causing much more injury and death than two vehicles travelling at 80 km/h and approaching from intersecting roadways. We then must consider, not only the Impact Speed of the vehicles but the Change-in-Speed, or more precisely the “Change-in-Velocity” of those vehicles as a result of the impact.

If Posted Speed, Travel Speed and Impact Speed were such complete predictors of injury and death, federal agencies such as the U.S. National Highway Traffic Safety Administration (NHTSA) or Canada’s Transport Canada would have used these parameters in their decades of inquiry into the mechanisms of injury. But that is not the case. On a general level NHTSA determined long ago that, over a broad number of collisions, the documentation of collision evidence for the purpose of calculating a Change-in-Velocity (or Delta-V) would be a useful exercise. Therefore hundreds of investigators spent numerous hours measuring the extent of crush on vehicles, estimating the stiffness of those involved structures and then calculating the Delta-V experienced by those vehicles. Upon completing that process investigators would also document the specific evidence of occupant contact visible in the vehicle interior or exterior. By obtaining reliable medical information on the specific injuries sustained by involved persons, investigators could then match the injury to its source. Now a second process of considering the “Second Collision”, that is the collision between the human body and the vehicle, could be begun. So not only is it important to calculate the Delta-V of the collision-involved vehicle, but the calculation of the severity of impact between the human body and the vehicle is equally important, if not more so. And, eventually, the calculation of a third collision, which is the collision of the internal organs of the body within itself is completed. It is the acceleration, or the rate of change in velocity, that describes how injuries differ from one instance to the next. And further, it is the rate of change in acceleration, or “Jerk” which has a further influence.

So, understand that Speed, in whatever way it is described, can be a useless value in understanding how and why collisions occur and how injuries and deaths occur. Speed has really nothing to do with injury causation or death except that, in a sense, Speed creates the potential for injury because it is what causes vehicles and persons to possess kinetic energy that must be dissipated. Dissipation in a controlled manner is what is desirable and this is accomplished in many ways through the design of vehicle structures, the use of safety devices such as seat-belts and air bags and through the design of the highway structures that extent the time of vehicle deceleration while also reducing the possibility of intrusions into the occupant space or reducing the chances of unpredictable chaos when vehicle rollover is initiated.

Yes, when an inappropriate Travel Speed is chosen by a driver that can increase the probability that a collision may occur. But that has nothing to do with what severity of collision will occur. There are far too many factors involved in this relationship to enable Travel Speed to be used as a useful predictor of injury or death.