Mark Twain included this delightful passage in his book Life on the Mississippi:
In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen.
It came to my mind as I drove south (the only lawful direction) on SW Broadway past Portland State University, where from Market Street onward the City has removed one of the three traffic lanes to put in a bicycle lane. The specific question that occurred to me was to ask myself when, at the rate that the City is removing traffic lanes from city streets, it would have no streets at all. I think we have a supply that will be good for several thousand years to come, but I don't know all of the lane-removal projects, and I could be off by a few centuries.