Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Spreading out wear on the roads

Learn more physics!

Most recent answer: 10/22/2007
Q:
Regarding our Permanent Lane Road System: I am attempting find the life cycle difference between roads where lanes are alternated, each year, to utilize the entire lane compared to the 50%that is utilzed by our current permanent lane road system. Traffic lanes are 12-feet wide and we drive over the same wheel positions until the road fails, which concentrates the weight of every vehicle into two 3-feet wheel paths in each 12-foot lane. My thought is: First, by utilizing 50% of each lane roads means roads will fail twice fast. (2x) This causes twice as many vehicles to travel over the exact same wheel position so that would accelerate road failure by another 2-times (2x) compared to utilizing the enitre lane. Then as the weight of every vehicle is concentrated into two 3-foot wheel paths in each 12-foot lane, the concentration of weight accelerates the wear process by at least 50%, thus roads fail (2x +2x +.5x = 4.5x) 4.5-times faster than they should compared to utilizing the entire lane. Is this correct? or would the Accelerated wear be greater? Thank you for your kind attention in this matter. Sincerely, Ron Moore
- Ron Moore (age 62)
Myrtle Creek, Oregon
A:
Yup, the roads wearing down and crumbling is a constant, expensive irritation. Congress just passed an approximately 290 billion dollar highway spending bill for fixing worn-out roads and building new ones. One might imagine that schemes designed to reduce the rate of road failure might be welcome.

Driving down most roads and looking at the pattern of wear on the pavement does indicate that a good fraction of the pavement rarely comes in contact with tires, and that most of the contact is restricted to wheel paths.

I must confess, though, that I am not convinced that a scheme of evening this out would either work or be cost effective or safe. Road failure comes in a great variety of forms -- potholes form not only in the places where the wheels come in contact but also in places where the cars don't press down so much. Part of the reason for that is that cracks may form in the part of the lane between the wheel paths which allows water in, which melts and freezes. Freezing water expands, cracking the pavement further. Evening out the wear might push cracks back together again, but I wouldn't be too optimistic.

High-traffic, hard-to-maintain roadways are built with better methods and materials than small roads and many city streets. A good, high-quality roadway is built with concrete which has reinforcing steel bars inside. This prevents cracks before they get started. It is also important to build a road on a stable foundation -- shifting earth can cause more problems than the traffic wear. Maintenance can help -- sealing the cracks with tar is a common way to extend the life of a road. A road which is poorly designed or constructed on weak soil which washes away in the rain (some roads in mountainous regions of California where mudslides are common come to mind) will fail regardless of wear schemes.

That said, there are additional problems with evening out the wear. A simple scheme may simply guide traffic to shift over by half a lane so the wheel paths run over previously unworn pavement. But roads are constructed so that cars which follow the normal wheel paths pass oncoming cars at a safe distance. You'd have to arrange so that the oncoming traffic shifts in the same direction, and make the roads clearly marked enough that people won't follow the old wheel paths but the new ones. Sometimes paint on the road isn't enough -- people also use roadside markers and the shoulder to calibrate where they should be driving. I've noticed this in construction zones where the lanes are shifted, sometimes by partial amounts, and sometimes it can be confusing to tell just where the right place to drive is.

You'd probably have to build a half-lane on the side to shift everyone over. Perhaps new roads can be built extra-wide for this (but then the temptation to just add another half-lane may be overwhelming).

Tom

(published on 10/22/2007)

Follow-up on this answer.