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OK, when I refreshed my memory by looking at a map, I realized that Elliott Bay and Lake Washington squeezing in in the middle is probably at least 60% of the problem, so I generalized it some, although it's now a little wordier. Basically, at least for a straight road (ie following the grid), it's pretty much limited to the area of the Montlake Cuta street between Lake Union and Portage Bay would run into the Duwamish, adding the cost of a second bridge.
And, indeed, if you include Lake City Way, and MLK Way, 23rd/25th does indeed come closest to qualifying (as connecting N boundary to S). (I don't feel strongly about having the explanation here, I just added it to answer the question that I thot many readers would have.)

As for Madison being the only E<>W, the offset downtown grid doesn't help, bodies of water play a part, tho' not as much (even tho' the Duwamish is generally narrow enuf for a bridge, there aren't going to be many built). Hills probably do make up most of the rest of the reason (at least besides the fact the probably no one really made it a goal). I assume the reason Madison does is related to the ferry it used to serve that crossed Lake Washington, tho' I don't know offhand which came first. (Yesler probably would also be one, except the for the hill on the east end.)

PS I like your additions to the layout article (I don't have any Seattle/King County street maps down here [but ordered a King Co Thomas Guide earlier today], so I was mostly going from memory about which cities, although I should have remembered Shoreline, since I lived there a few years, and Mercer Island, since my dad worked there about 20 yrs), and THANK YOU for fixing ""road label""I knew it was bad, but was having a mental block as to how to fix it. Oh, and leaving out ""numbering"" there is fine.

PPS Turns out the road preceded the ferry, which wasn't planned at the time, so they're less connected than I thot. I guess the reason is that the street was made by someone that wanted the connection, and owned enuf land he could do it, and did it back when it was still possible; probably a rare combination.  20:20, 8 Apr 2004 (UTC)"