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Oh, Almighty Matt: I noticed a great dip in the fast lane about half a mile from the I-5/805 split. What's the reason for that? Did they run out of dirt to fill in the dip?

-- OGS, the Net

Dear M.A.:

Most San Diego interstates are concrete, but not Hotel Circle. Is it asphalt there in order to reduce the noise, for the benefit of the hotels? If so, how did this arrangement come about? And since I assume asphalt must be repaired and repaved much more frequently than concrete, who pays for the extra maintenance?

-- Looking for Concrete Answers

Hey, Matthew:

What's the scoop with the lane markers/reflectors on I-8? They're above ground until you hit the mountains. Then they become subterranean.

-- Ben Davis, San Diego

In spite of the fact that we mugged Caltrans for a year's worth of answers, they gave us a royal reception. None of our queries contained the words "Murphy Canyon sinkhole." They were glad for the break. And as usual, they had all the facts. For OGS: The I-5 dip is nothing compared to the roller-coaster ride on 52, between 15 and 805, south of Miramar. The cause of most dips is the soil gradually compacting under the road surface for a whole range of reasons-- groundwater changes, runoff, erosion, stuff like that. Since Caltrans examines what's under a freeway before they pave it, the subsidences are usually no surprise. It's just a matter of periodically adding asphalt to the dip to smooth it out, a cheaper solution than rerouting the road. That was certainly the case with 52, which was built over the old Miramar landfill. As we drive across it, we crush our old mattresses, apple cores, egg shells, creating dips that actually are kind of a rush if you hit em going 70, 80. Of course, my attorney and I are not suggesting you try it. And not that I've tried it myself. I've just heard tell.... Anyway, live with the dips. They saved us a chunk of road-building change.

I like a good conspiracy as much as the next guy, Mr. Concrete. But I don't think there's any funny money changing hands on this one, even though, yes, concrete is more durable than asphalt. Hotel Circle blacktop has nothing to do with tiptoeing past the tourists. If we wake em up barreling to work in the morning, well, too bad. Take a clue from the previous answer and consider that any road surface laid in a soggy flood plain needs some flexibility. That means asphalt, not concrete. And any question that begins, "Who pays for..." doesn't really require much head-scratching. Meditate upon the check you wrote the DMV to renew your plates and perhaps the answer will come to you.

As for Ben's buried lane markers, imagine this. Big snowfall. Big snowplow. Blade scrapes along road surface. Above-ground lane markers pop like bodice buttons in a romance novel. Caltrans countersinks the markers at elevations above the snow line, about 3000 feet.

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