Scratching below the surface of the pavement

By : 
Reflections from my Notebook

I was with a group of people discussing the recent completion of Fillmore County Road 1, which is now concrete, after three years of construction. Once we went over the wider sightlines, smoother surface and milder curves, the conversation turned to a rather obscure question — whether concrete or asphalt provides the shortest stopping distance.

No conclusion was reached, so someone suggested I write about it. I don’t normally take requests because columns that come from within me are much stronger — and more fun to write. However, the inconsequential question was intriguing as it was still on my mind several days later. I had previously written about the comparison between concrete and asphalt regarding longevity and cost, but this was a new angle and I love to explore, or play with, ideas.

Most people probably wonder why I would even consider this issue as worth my time since a paved road is a paved road. However, another reason it interests me is that my extensive long distance running has shown that there are significant differences in pavement, even if they aren’t apparent.

Runners generally avoid concrete surfaces if possible because they are hard on the feet. If you don’t believe that is true, one runner author recommends you hit the two surfaces with a hammer to see how it feels on your hand and arm. The asphalt will slightly dent, but not the concrete.

With thousands of steps in each run, the impact on the feet can really add up. A study shows that feet strike the surface with a force of up to six times a person’s body weight.  Over many miles, the constant pounding on a surface that has some give compared to rigid concrete with no give can make quite a difference to the small area that takes the impact of each foot-strike.

The people I was with when County 1 came up are drivers, not runners, so the intricacies of shoes hitting the pavement weren’t as interesting as how different pavements affect the ability to stop a car.

The answer to that question, like many technological and scientific analyses, is a lot more complicated. It is difficult to find a decisive conclusion, perhaps because the differences are more dependent on other factors and road surface in the abstract plays a very minor part in stopping distance.

In fact, many of the facts used to support the safety of one surface came down to other factors, such as durability and maintenance.

One argument for concrete providing the safest driving surface is that it is rigid, so no ruts are formed. During wet weather, water or ice collected in the ruts of asphalt pavement can increase the potential for skidding.

However, the same factor can be used to argue the other way. Because asphalt wears down faster, it has a rougher surface, which is good for traction. And, as for the ruts, well there is an argument made that they allow better drainage, creating safer roads during bad weather.

On the other hand, when asphalt roads really deteriorate, potholes create many more safety issues than stopping distance.

Still, our discussion was on new roads and I assume the question was asked about the difference now rather than at some time in the future. The scientific measurements I found show a very slight difference between the two. Dry concrete gets the edge over asphalt, but wet asphalt gets a slight edge over wet concrete. The differences are very minimal, though.

As far as measurements that translate more to real life driving, an online calculator provided by an engineering consultant shows the stopping distance for a car traveling 50 miles per hour on asphalt is 119.47 feet compared to 104.53 on dry concrete, making concrete the winner. However, change the conditions of each surface to wet and the stopping distance is the same.

More interesting is the big difference when there is a change in the conditions to snow, which more than doubles the stopping distance on either surface. And, ice doubles that distance again.

Those results show the most important conclusion of all. The type of road surface isn’t nearly as important as other factors, and not just those that are based on the impact of weather.

For example, reaction time is key to stopping distance because that is the first thing that happens when a driver has a reason to stop. With so many things to distract drivers today, slow, or no, reaction time is more of a problem than a tiny difference in the makeup of a dry road surface.

The quality of tires is also a bigger factor than pavement surface. Although tires have become more sophisticated so that we may not worry about them, improperly inflated tires extend the stopping distance. Properly inflated tires maximize the tread contact with the surface, thus providing more safety.

Rate of speed also plays a big role in stopping distance. Increasing the speed in the example given from 50 miles per hour to 70 nearly doubles the stopping distance. Also, slowing down in poor conditions have a great impact. Driving at 25 miles per hour on snowy roads decreases the stopping distance to well below that when driving 50 miles per hour on dry roads.

So, there you have it. The obscure question led me on a wandering journey, but in the end I hope you got some useful information out of it.