In Part 1 we gave a quick refresher on stability control and collision mitigation (CMS) technologies, laying out the similarities and differences between driver control and system control in a collision situation. We also touched on the basic fact that stopping on a slick surface requires more time and more distance to bring an 80,000 pound tractor-trailer to a stop.
Before we get started with Part 2, let me share an important point to keep in mind about these driver assistance technologies. Bendix® safety technologies complement safe driving practices. No commercial vehicle safety technology replaces a skilled, alert driver exercising safe driving techniques and proactive, comprehensive driver training. Responsibility for the safe operation of the vehicle remains with the driver at all times.
Now, let’s answer the two questions that started this conversation, “What can I expect from collision mitigation on slick surfaces?” and “Won’t automatic brake application cause me to lose control?”
To help answer these questions, I went to our winter test facility at the Keweenaw Research Center (KRC) in Michigan’s upper peninsula. I always enjoy the opportunity to learn from our engineers as they conduct another year in their ongoing technology testing program. The approach to driving was simple – allow the system to apply the brakes to help mitigate a potential rear-end collision on a stationary, or parked, vehicle in our lane of travel, a maneuver that is often done at our live demos (and one that our engineers frequently use while testing our systems). This time, however, instead of driving on a dry or damp surface, I’m running it on a roadway surface of packed snow. By doing so I can compare the results of the system braking to what I’ve experienced on a dry surface.
For the situation, a 6x4 tractor with a loaded trailer, air disc brakes, and a gross vehicle combination weight of 65,000 lbs. was used. The speed of the combination vehicle approaching the stationary vehicle (which was one of our inflatable test vehicles) was between 30 and 35 mph.
As I was driving it was obvious that the collision mitigation system (CMS) intervened as it was designed to do – alerting, cutting throttle, and applying the brakes in an attempt to help the driver – in this case, me – potentially mitigate or reduce the severity of, the collision. However, because of the slick surface, the system - just as if the driver applied the brakes - needed more time and distance to mitigate the crash than on a dry surface.
Next, the team and I took another pass at it with the same combination (tractor-trailer weighing in at 65,000 lbs gross vehicle combination weight) approaching a moving target that was going slower than we were. In our demos, we do this maneuver – we call it the rapid approach – to emulate that age old situation most drivers have encountered on the freeway, the slow-moving vehicle in your lane of travel.
Again, traveling on a packed snow surface, the tractor-trailer approach speed went from 30 mph to 45 mph in increments of 5 mph. The forward vehicle speed was traveling at about 10 mph. As with the stationary vehicle, the system alerted and applied the brakes. And, as before, while I slowed, I did not achieve enough speed reduction to avoid hitting the vehicle. Instead, I was able to swerve to avoid the forward vehicle and, thus, avoid the crash.
In other words, the system reacted as it was designed and allowed me the opportunity to take action to swerve to avoid hitting the car ahead.Not all that surprising. Whether the forward vehicle is moving or stationary, bringing a loaded tractor-trailer to a stop, , requires more time and distance on a slick surface. How much distance? Let’s take a look.
First, start with the coefficient of friction (CoF). CoF is a measure of the amount of friction existing between two surfaces. A low value of coefficient of friction indicates that the force required for sliding to occur is less than the force required when the coefficient of friction is high. Friction for our consideration ties to tires on the road surface. Slick surfaces have low CoF and dry surfaces have high CoF. Interestingly, the difference in the coefficient of friction of truck tires on a hard-packed snow or an ice surface compared to the CoF of truck tires on a dry asphalt or concrete surface is up to 87% less
Second, considering the mass of the vehicle – let’s use 80,000 lbs. for a combination vehicle – and a speed of 55 mph, we can roughly calculate the stopping distances on various surfaces:
- Dry surface = ~ 335 ft
- Wet surface = ~466 ft
- Snow (packed) surface = ~965 ft
Ice surface = ~1,625 ft
Source: HPWizard.com "Tire Friction and Rolling Resistance Coefficients"
It takes almost 3x the distance to bring the vehicle to a stop on packed snow vs. a dry surface. And almost 5x as much on ice. This means both the driver and the system need to be aware of what is happening down the road in order to react in time.
All the more reason why an alert driver is still critical. Drivers can see farther than systems (unless it’s blowing snow, which raises another issue of whether or not it’s safe being on the road, but we won’t get into that here!). Drivers can also steer, something that most of the collision mitigation technologies on the road don’t do today.
The value of the collision mitigation system on a slick surface is not only found in the intervention, but the alerts as well. Depending on the situation, the CMS alerts can help re-engage the driver to brake or take other action to avoid a hazard. If the driver intervenes to steer around the situation, stability control can help them keep control of the vehicle.
As to the potential loss of control caused by system generated collision mitigation braking; this is no more of an issue than the driver braking on the same surface. In my driving examples, the collision mitigation system braking did not result in the combination vehicle losing control. And even if it had, the electronic stability control system could’ve helped me recover.
But of course, no driver should rely on the collision mitigation and stability control systems to bail them out of a situation – the person behind the wheel is always in control. While it’s good to know the systems are there, a safe, alert driver practicing safe driving habits and maintaining control of the vehicle, is absolutley essential to mitigate a potential collision,– whether on a slick or dry surface.
So to wrap up, collision mitigation system braking is comparable to a driver braking on a slick surface. More importantly, safe, alert drivers are the single most important factor critical to help mitigate collisions in inclement weather. Keep in mind – the driver does have advantages over the collision mitigation technology – sight and steering. What I said at the start of this blog bears repeating: Bendix® safety technologies complement safe driving practices. No commercial vehicle safety technology replaces a skilled, alert driver exercising safe driving techniques and proactive, comprehensive driver training. Responsibility for the safe operation of the vehicle remains with the driver at all times.
Time and distance are key. Increasing distance usually means an increasing safety margin. Therefore, backing off is important anytime, but even more critical when the weather gets bad and the road gets slick.
Technical and industry insight from OUR experts.