Dark spots on I-5 are fog seal

While driving on Interstate 5 recently between Medford and Ashland, I noticed the roadway surface appears darker, as though someone had sprayed something on it. Is the roadway going to be resurfaced again, or has some kind of preservative been applied to extend the life of the roadway? Also, the new asphalt already looks like it's getting shiny in areas, like it's wearing out.

— Jim S., Phoenix

You've made a number of good guesses about the mysterious dark matter deposited on the freeway, Jim.

Your initial guess is correct. A spray was applied to the surface to extend the life of the asphalt by about three years. Known in the trade as a "fog seal," it is a mixture of oil and water. The water evaporates fairly quickly, and the oil solution bonds with the rock that is embedded in the roadway, according to the Oregon Department of Transportation. The fog seal, applied in late August at a cost of $200,000, was sprayed from Exit 19 in Ashland to Exit 35, north of Central Point.

The road was paved about five years ago, and there are sections where the oil in the asphalt bleeds to the surface over time, creating the shiny patches. ODOT anticipates the roadway will have a life span of about 12 to 15 years before it is paved again.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by e-mail to youasked@mailtribune.com. We're sorry, but the volume of questions received prevents us from answering all of them.

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