Explanation – Speed, Difference in Speed and Change in Velocity
Travel speed, change-in-velocity and road safety are very different concepts that need explanation
It is rare that anyone provides the public with a simple explanation of the difference between “speed of travel along the ground” versus “difference-in-speed” or “change-in-velocity”. These concepts are continually confused yet they are very important in understanding collisions and road safety. Let’s look at some theoretical examples.
Two vehicles travelling in the same direction, side-by-side, along an expressway at 110 km/h may be involved in a collision when Vehicle #1 drifts into the path of Vehicle #2. Since their forward speeds are the same the primary factor that determines the severity of the collision is the speed at which the drifting Vehicle #1 moves toward the side of Vehicle #2. One can visualize this as a minor incident, setting aside what complications may develop after this initial contact.
In a second scenario imagine Vehicle #1 is no longer drifting but is actually travelling behind Vehicle #2 and Vehicle #2 is now travelling only 90 km/h. Vehicle #1 travelling at 110 km/h rear-ends Vehicle #2. Even though Vehicle #2 was travelling slower the severity of the impact was higher to both vehicles because the difference in speed of the two vehicles was higher. The same concept exists at much slower speeds.
Now imagine a city street where Vehicle #2 slows to a speed of 20 km/h and is rear-ended by Vehicle #1 which is travelling 50 km/h. The speeds along the ground are much lower that the examples on the expressway (discussed above). Yet the collision severity is higher in this city street example. Why? Because the difference in speed is higher. In our city street example the difference in speed is 30 km/h (50-20 = 30). In the expressway examples, the difference in speed of our rear-ending example was only 20 km/h and the original example of the drifting vehicle would involve a difference in (lateral) speed of only a couple of km/h.
It is the difference in speed that determines collision severity and what injuries may occur. There are other complications in this discussion which will be left out for now. But one additional explanation will be given.
Speed is a concept that has no “direction” attached to its definition, it only provides a number describing a magnitude. A concept that provides a fuller explanation in collision analysis is the concept of “velocity”. Velocity is the same as speed except that it has a direction attached to it. So a speed along the ground may be 30 km/h, whereas a velocity along the ground may be “30 km/h north”. In the field of road safety research and collision reconstruction velocity is the important concept.
Researchers and reconstructionists often use the following concepts interchangably: Collision severity, Change-in-velocity and Delta-V. They are really talking about the same thing. We often define the severity of a collision by the change-in-velocity caused by an impact. Thus in the city street example, where Vehicle #2 travelling at 20 km/h is rear-ended by Vehicle #1 travelling at 50 km/h, the severity of the impact may be defined by how much velocity was lost during the time of contact. If both vehicles are of the same weight, of infinite stiffness and moving on a frictionless surface they both might sustain a change in velocity of 30 km/h. That is, in a theoretical scenario, Vehicle #2 (the struck vehicle) might be propelled forward to a speed of 50 km/h whereas the velocity of Vehicle #1 may be dropped to 20 km/h. But such an example will never occur in real-life and something more realistic might involve both vehicles exiting the impact at a velocity change of 15 km/h or less. When we say that a change-in-velocity of 15 km/h has occurred we often exchange this wording to say that a “Delta-V of 15 km/h” has occurred. The Greek letter “Delta” replaces the words “change in” and the “V” replaces the word “Velocity”.
Things get more complicated when we look closer at the time of contact and what forces and accelerations may exist. A Delta-V of 15 km/h may be quite different when the time of contact is very short versus when it is elongated. We often say that in a severe head-on impact the Delta-V may be completed in just over 100 milli-seconds. 100 milli-seconds is the same as a 10th of a second. But there are many intersection collisions where the Delta-V may be completed in 300 milli-seconds, thus reducing the forces and accelerations at any given time. Yet, in a head-on collision there is much more length of crushable structure that can protect an occupant whereas a vehicle struck in the driver’s door has much less protective structure. So the comparisons and complications can continue with many additional explanations being required.
But keeping the discussion simple, we need focus on this singular difference between ground speed and difference in speed or Delta-V. There are many times in public discussions when this becomes important. The most common discussion revolves around comments that, if we only reduced the posted speed limit along some section of road then our safety problems would be solved. The obvious fallacy is that changing a posted speed does not mean that the travel speeds will change. Even if the average travel speeds are reduced there is no guarantee that the target speeder’s actions will be changed, it only means that the general, law-abiding, road-user’s speeds will change. The speeder that was the original cause of the problems may continue to ignore the newly reduced speed just like he ignored the original, higher posted speed. But with respect to ground speed, the danger of the severity of a collision is not how fast a vehicle is travelling along the ground but what change-in-velocity is caused during contact. We need to remember from the above examples that a vehicle travelling at 20 or 50 km/h may be involved in a more severe collision than a vehicle travelling at 110 km/h. We need to understand when these concepts apply and when they are being used to confuse an issue.
Better Access to Collision Data Needed to Properly Inform the Public
Publicity surrounding a sudden rash of fatal collisions in the last couple of days can be confusing. Does it mark an important change in pattern or is it simply due random fluctuation? These are the kinds of questions that could be answered by providing the public with better information about collision trends and statistics.
A sudden rash of motorcycle fatal collisions in last couple of days needs to be placed in its context of historical trends.
OPP collision data has been provided for the first 6 months of 2019 indicating 33,839 crashes, comprised of 4272 injury and 118 fatal. However changes have occurrred in reporting collisions such that many “minor” collisions are not reported. And incentives not to report injuries make it possible for injury collisions to become unreported. There is also an increased trend to report fatal injuries as due to pre-existing medical conditions such as heart failure and therefore not included in collision statistics.
Similar data reported for motorcycle collisions indicated that in the previous 11 years prior to 2019, there were an average of 30 motorcycle deaths whereas, up to the current date there have been only 11 deaths. According to these numbers fatal motorcycle collisions might be much lower than average in 2019. Yet, with the small numbers involved, the deviation in those numbers could be substantial from year to year or season to season.
Many important issues remain hidden in the general numbers. Numbers of persons drowning when vehicles enter roadside waters are not provided. Incidents of vehicle fires are also kept from the public’s knowledge. Incidents of vehicles striking roadside buildings may be related to “sudden unintended acceleration” that may be malfunctions of vehicle manufacture or maintenance but these occurrences are also not tracked. Incidents of unexplained major injury or death are never revealed as such in the public domain. Roadway safety problems such as surface conditions or roadside malfunctions of infrastructure are rarely revealed.
In all these matters, better collision data are needed to be provided in the public domain so that the public can decide for itself what is important and where the issues of importance truly lie.
Deception in Median-Cable-Barrier, Multi-Fatal Collision on I-85 in Georgia
Apparently it does not take much effort to hide the fact that a median cable barrier failed to prevent an SUV crossing a median and killing 7 persons. While multiple news coverage focused on small images of the final rest position of the involved vehicles, attention was taken away from the tell-tale status of the damaged cable barrier.
These facts revolve around a tragic, multi-vehicle collision on I-85 north-east of Atlanta Georgia on Saturday, July 6, 2019. A northbound SUV reportedly crossed the median for unknown reasons and entered the southbound lanes. The resulting collision with two southbound vehicles resulted in the multiple deaths. The fact that all 4 occupants of southbound van were killed is another peculiar occurrence that needs further inquiry, but at another time. What is more revealing is how the public’s attention was diverted from the issue of the performance of the median cable barrier. Since no one was interested in documenting the poor performance of the barrier there are no photos available except some poor quality images that can be obtained from video at the site. Never-the-less, what poor quality images remain provide sufficient documenetation of the cable barrier’s failure.
The still image shown below is a frame taken from video of the site. While the focus of the video was with respect to the vehicles in the distant background, the important information lies within the red circle where we point out the status of the cable barrier.
In this video frame the public’s attention is taken away from the important fact that the top cable of the median barrier is slack while the bottom 3 are tight.
Firstly, we can observe that the cable barrier exists only on the far side of the median, much like the situation that exists along Highway 401 between London and Tilbury, Ontario. Secondly, all the news reports simply confirmed that a Ford Excursion SUV crossed the median and nothing more. Yet, is it the purpose of the cable barrier’s existence to prevent such a cross-over? So why would news media not ask this simple question: How and why did the Ford SUV cross through the cable barrier when that should not happen? Is that not an important question when the result is that 7 innocent persons lost their lives?
The video frame below shows a closer view of the cable barrier. Note how the top cable is slack while the remaining three are tight.
A closer view of the median cable barrier shows that the top cable is slack and the remaining three are tight. What does this say about how the Ford Excursion passed through the cable barrier?
Did the Ford Excursion travel over top of the cable barrier? Surely there is damage visible to the anchorage posts of the barrier in the extreme top corner of the video frame. So some contact was made with the cable barrier.
The warning signs should be raised when a similar occurrence was documented by Gorski Consulting on December 11, 2018 on Highway 401 near Merlin Road, between Windsor and Chatham, Ontario. The photo below shows the aftermath that fortunately, and miraculously, did not result in a fatality.
This collision on Highway 401 near Merlin Road occurred on December 11, 2018 when an eastbound truck passed through the median cable barrier and struck westbound vehicles.
Again, little attention was paid to the fact that the cable barrier did not prevent the truck from passing through the barrier. A TV productrion crew was allowed onto the site to film the events for an upcoming television program. While the film crew covered the vehicle damage and the actions of the towing operators, the important matter of the barrier appeared to be of no interest.
While a film crew’s video was focused on the damaged trucks, behind them was a damaged cable barrier that received no attention.
A wider view of the Highway 401 site is shown below. Again, much like the Georgia collision site, there was a cable barrier only on the far side of the median, not on the near side.
View looking through the eastbound lanes of Highway 401 and into the median showing that the median cable barrier only existed on the far (north) side.
While an investigator with the University of Western Ontario Multi-Disciplinary Accident Research Team in the 1980s, Zygmunt Gorski conducted a number of detailed documentatiuons of the travel paths of vehicles passing through the median of Highway 401 before a concrete barrier was installed. From this activity it was obvious that such travel paths and vehicle motions were chaotic and generally unpredictable when the vehicle reached the far side of the median. Even a median that is orginally designed with a well-groomed and relatively controlled slope will eventually take on uneven characteristics over the years. The belief by roadway designers that they can predict the path of a vehicle through such a median surface is clearly unrealistic. Once a vehicle reaches a barrier at the far side of the median the manner in which it strikes that barrier must be understood to be very unpredictable. And this is exemplified in the two examples shown above.What should be of concern is that, once again, little attention is being paid to these important matters that threaten the lives of innocent persons who have a right to arrive at their destinations in safety.
Highway 401 Rear-End Impact – Why Was A Truck Travelling So Slowly in Fast Lane?
Simply blaming the rear-ending truck driver for a severe impact will not protect anyone from the next tragedy. But an objective, detailed analysis will. Scientific study can illuminate unknown or hidden factors that might be corrected resulting in a reduced chance of a future occurrence.
A serious, rear-end impact occurred in the early evening of July 5, 2019 in the eastbound passing lane of Highway 401 at the Iona Road interchange south-west of London, Ontario. The Ontario Provincial Police (OPP) displayed two photos of the two involved transport trucks on their Twitter account. These photos are shown below.
This OPP photo shows gouges in the foreground of the passing lane of Highway 401 where the initial impact occurred, and the two transport trucks stopped at the Iona Road overpass in the background.
This OPP photo shows the extent of frontal crash of the cab of the striking truck as it collided with the rear of the trailer of the other truck. The extent of structural intrusion into the occupant space of the crushed cab would be the primary, injury-causing factor as both occupants of the cab reportedly sustained life-threatening injuries.
It is estimated that the trucks travelled approximately 40 metres from impact to rest. A truck skidding from maximum brakiing would likely lose a speed of about 70 to 80 km/h in a distance of 40 metres. However It would be difficult to imagine that, if the first truck was fully-stopped, it could be pushed forward that distance without sustaining more visible crush to the rear of the trailer. Also, although the extent of crush to the cab of the striking truck is extensive, it does not appear to match the scenario of the truck impacting a stopped truck at a speed of 105 km/h. Thus the most likely scenario is that the first truck was likely travelling slowly in the passing lane when it was struck by the impacting truck which had already lost a substantial amount of speed before impact. This is the best that can be accomplished given that we do not have access to any site measurements or the full police photos that would normally be available in an official reconstruction.
Not unexpectedly, police and official news media provided only a scant reporting of the incident without the details that would be needed to understand how and why the collision occurred. What is known is that such collisions are not rare. In fact, in the recent history of the highway, they are becoming common. As drivers become accostomed to travelling long distances at high speeds without interruptions, a sudden slow-down of traffic is unexpected and drivers are not prepared for this violation of their expectations.
The common approach observed in many of the comments that appear on various public forums is to make judgments on the “stupidity” (and similar comments) of drivers who cause such crashes. Rarely are such comments accompanied by any detailed analysis or understanding of how and why the collision occurred. In many instances, such detailed analyses uncover important clues and result in better understanding of why such a collision occurred. Yet it is practically impossible for anyone to make such an evaluation because the base evidence is never available.
Trucks travelling this route may come from any part of Norh America, including Mexico. Attempts to improve driver habits often include publicity of police enforcement efforts on various local news outlets. Unfortunately these attempts can be of minimal use when drivers are passing through from vast distances where local news is not available to them.
While traffic slow-downs and stops may sometimes be unavoidable, they also need to be monitored and studied. One of the difficulties that shows on a regular basis is that drivers do not apply braking early enough, or hard enough, when it is needed. Lack of visibility ahead may be one of the crucial factors causing this problem. Large truck trailers become the walls that block the view of drivers of the road ahead and many drivers are accostomed to travelling very close to the rear of such trailers thus increasing the significance of the problem.
Automatic Emergency Braking (AEB) that is becoming more common on light vehicles is not likely to be installed on large trucks in the foreseeable future even though this technology is greatly needed. Thus, until that occurs, an important matter is to document those instances where traffic is slowed or comes to a stop and to evaluate whether there is something that can be done to warn drivers of that unexpected happening. Unfortunately actions such as these are not being discussed and serious rear-end collisions continue to occur without much public attention.
Connectivity, Artificial Intelligence & Privacy – A Tightrope Walk With Real Consequences
While there is a leeming’s rush to be ahead of the pack with development of large-scale, connected, artificial-intelligence networks, society’s privacy cannot be left to unregulated peeping toms at our bedroom windows.
In the field of road transportation there are obvious advantages to connecting vehicles to each other and to the roadways on which they travel. However there must also be a framework that protects the personal information that, inevitably, becomes gathered in the vast data networks needed for connectivity.
For decades the Ontario Minstry of Transportation has been monitoring vehicular traffic along the major, 400-series expressways on a 24/7 basis. The quantity of such data is mind-boggling. Similar data is collected from any large retail outlets that track purchasing actions through the use of debit, credit and loyality cards. Even individuals who use their cell phones and fitness meters are tracked in terms of their locations and habits. A real controlling factor in usage of this data has been the lack of computering power and artificial intelligence that is now being unleashed.
Google’s Sidewalk Labs in Toronto has proposed that collected data be overseen by an independent “data trust”. But, as has been seen before, lack of transperancy allows governments and large international corporations to infiltrate such agencies with partisan entities without the public’s knowledge, resulting in no meaningful trustworthiness.
Regettable losses due to the increased powers of new technology could be those simple acts of individuals who have no ability or intention to use personal data inappropriately. Persons taking videos of their families in public places used to be an innocuous act. Various data that is unconnected and created independently in small envolopes used for various private and public purposes may also be a new target. The possession of independently-developed information itself may become illegal under new laws that attempt to harness the vast powers of large, private, data-analysis corporations and, in the process, create a society of “1984” depicted in literature and “2001” in film.
While connectivity, big-data analysis and artificial intelligence have the potential of providing great benefits to society, they also provide the ability for large, and small, local and international, entities to escape detection and oversight. As developers of these sophisticated systems we have the capability to study the horizon ahead before following our hysterical brethern over the proverbial cliff. Let us hope this remains our choice.
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