In 1964, AASHTO abolished US route suffixing. In 1980, they abolished suffixing except for "divided routes" like the 35s in the Twin Cities and Dallas/Fort Worth and I-15E in Riverside.
Why? "Route confusion". Actually, AASHTO created that itself. I-76 once was "I-80S". I-84, at least on the West Coast, was I-80N (and unfortunately, a 5th grade level "Map Skills for Today" workbook by Weekly Reader, an environmentalist-biased publication labeled that as I-80 mainline). I-270 was I-70S, and I-70 was I-70N. I-680 near Omaha was I-80N, which IMO is the most blatant example of AASHTO's hogwash.
It's kind of ironic that this "spur suffixing" was not as prevalent on the US routes, which had them canceled in 1964, as were the Interstates. 70 and 80 seemed to have the worst examples. Why does this cause route confusion? Simple: Let's say a person from New York City was heading off to San Francisco, and near Omaha he sees a sign saying "I-80N to I-29". That person will think that I-80 continues and the N route is simply an alternate. Of course, I-80N ended at I-29.
There are many places where "loop suffixing", or suffixing divided highways are very useful. The best examples would be places where the mainline bypassed the city, and the through-city route was straight enough to be a good "through" route, notwithstanding congestion. (Sorry I-279, drivers must go east on 22, so it wouldn't deserve a suffix.) Examples of this would be I-215/I-15 in Riverside,California (in fact, I-15E was on I-215 until 1983); I-275/75 in Tampa/Saint Petersburg, and even I-475/75 in Flint, MI (that's extreme; through traffic wouldn't benefit). I-95 and the NJTP would benefit with this arrangement as well; an I-95E for the NJTP route south of Exit 6 and also the E/W for the East Leg/West Leg near Secaucus.
The advantages: Another alternate route for through travelers...those changeable message signs with the LEDs could be useful for them, as well as local commuters. Would allow services on both sides of the route to benefit. Would alleviate traffic if "induced demand" does not already exist. Would end some confusion about "3dis aren't the main route, I'm just taking the main route".
Disadvantages: AASHTO unwillingness, ad confusion (Dallas/Ft. Worth ads regularly miss the suffixes)...
Verdict: As I said before, if done properly, suffixes will be useful for through travelers, and for cities.
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