Does passenger car traffic actually damage roads?

Question

Pavement Design

In roadway pavement design, the (US) controlling design load is usually the Equivalent Single Axle Load (ESAL). This number attempts to quantify how much bending stress a given vehicle causes in the pavement. Trucks have a much higher ESAL load than cars. In this case "truck" means "semi, big rig, freight carrier" and not a family pickup truck.

Typical values are:

• Car Axle = 0.0002 or 0.0003 ESAL
• Legal Truck Axle = ~2 ESAL (wide range based on weight)

By this measure, a typical car or pickup truck doesn't place any appreciable stress on the road pavement.

Pavement Life

Two sections of road that are designed and built exactly the same and a certain period of time. This period of time could be 5 year, 10 years, 20 years, etc.

1. Road driven on only by typical family or commuter vehicles. Basically only passenger vehicles (not busses) and not hauling goods or construction materials.
2. Road with no vehicle traffic, but experiences the same weather conditions as (1). This would also include salt application in areas where that it common.

Questions

Would you be able to tell which section of pavement was driven on?

Would light weight vehicles cause so little additional damage to the pavement that it would be indistinguishable from natural weathering?

The corollary is that trucks cause all of the pavement damage.

Just to be clear, I am not concerned with visual things that you would be able to use to differentiate, e.g. oil stains, rubber marks, etc.

Answer 1

Yes, there would be a noticeable difference. Normal passenger cars don't really stress the road surface much. That's not so much the issue. As a analogy, you can have 100 people walk across a small plank, and it holds fine, but even 10 people on the plank at the same time will stress it to breaking.

Cars in normal operation don't much hurt a normal road surface, but the road surface will degrade on its own. Eventually a point is reached where normal cars cause significant additional damage. For example, consider a small rock that is part of the asphalt mix coming loose over time. Cars driving over it can dislodge the rock and randomly move it around until it ends up on the side of the road. Without cars, the loose rock would have stayed in place, at least a lot longer.

This small rock principle multiplies over time. A dislodged or two small rock creates a weak spot, so ordinary car traffic can accelerate more material getting dislodged at the boundary. This causes the boundary of a defect to grow more rapidly with car traffic than without.

Of course there is a flip side too. Car traffic kills plants growing in cracks. Any abandoned road, even after just one growing season, will have weeds growing in cracks. Cracks are inevitable, and so are wind-blown or bird-dropped seeds. Roots from these plants grow, dislodging road material, and trapping windblown dirt, making more soil. This causes more plants to grow, more soil to be made etc. In places with enough rain to be generally green if left alone, a road disappears under soil remarkably quickly if left alone.

So, the difference will be quite noticeable because completely different forces are at work tearing down a active road versus a abandoned one. The main state road near my house in Massachusetts was last repaved 20 years ago. It now has lots of cracks and has developed lots of potholes in the last few weeks due to the spring conditions. However, I've seen roads abandoned for only 10 years hardly look like a road anymore unless you are on top of it and see the patches of asphalt in between the significant clumps of weeds.