By Erik Hare
You drive on the street in front of your house every day, but how often have you looked at it? Perhaps on a sticky hot day a crew comes through, spreading the scent of tar low and wide across the street. Beyond those days, you might rarely the street where you live anywhere near as much though as the people and houses that live along it. No matter what, though, your taxes paid for what the Public Works Department has given you. What did you get?
Most of the streets in Saint Paul are not, technically “paved”. That term refers to streets that have been set with asphalt curb to curb, and in Saint Paul that only happens on major streets (on the left in the composite picture). The neighborhood streets are done with “chip and seal”, or crushed rock that has some oil applied to keep it together (on the right). You can see in the picture that on the right it is mostly rock but on the left it is mostly not-rock – pavement is mostly tar, not stone. What’s the difference?
It all goes back to the first paved streets in about 1820. John McAdam discovered that if you seal the water from below a road, you didn’t have frost heaving and other things that break up the surface. His method of sealing the road was to put down sharp crushed rock that was compressed to make a “macadam” surface. It worked. Our neighborhood streets are a simple variation on macadam where the stone is put down, compressed by cars running over it for a few days, and then sprayed with an oil that seals it all up.
If you ran this kind of surface with tar, you’d have a “tarmac”. That’s what many of the main roads are in Saint Paul, and what is pictured. You can also make a surface that is as smooth as glass with nothing but tar, but it doesn’t wear as well as a tarmac paving. Our Public Works Department prefers the tarmac type of paving just for the durability. Since they spend $28 million a year on "right of way maintenance", every small savings adds up.
There are problems with the “chip and seal” or sealed macadam kind of paving we use on our neighborhood streets. For one thing, they do require maintenance every few years to keep cracks like the one I pictured from forming. When cracks do develop, they often widen and spread much quicker, as you can see in the composite photo. But this kind of surface is still much cheaper in a place where there isn’t a ton of traffic forcing high maintenance, so it’s the choice for our neighborhood streets.
The way our streets are made is easy to ignore, if you choose to. That’s by design. We have a good Public Works Department that does its best to stay on top of things. It’s good to know what they are up to if, for no other reason, than we’re paying for it. I think we’re getting a pretty good deal in Saint Paul, too.
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