Major US routes vs. major Interstate routes.

[hbelkins]

Growing up, I knew that the major Interstate routes end in 5 or 0 because that factoid was included on many road maps. It wasn't until I discovered MTR that I learned that for US routes, the main routes ended in 1 or 0, not 5 or 0.

So when the Interstates were built, why weren't the major ones built parallel to the major US routes?

Some of them were, such as US 1 and I-95, but many weren't. US 11, the second major N-S route, parallels only I-75 for a short distance (and I-20 and I-40, too, if you want to get technical) but spends most of its time alongside 59 and 81. US 21's companion route was I-77, not an x5. US 31 is completely independent of any Interstate north of Indianapolis. Ditto US 41 between Nashville and Chicago. US 51 parallels I-55 only as far north as Memphis, and US 61 picks up from there but only to St. Louis.

And the same thing happens with E-W routes. Look at US 30, US 50, and US 60.

Had travel patterns changed in the years between the creation of the US route system and the Interstate system to the point where the major corridors were no longer thought to be the same? What was the reasoning behind designating different major corridors between the two systems?

[SkyPesos]

I take the x1 major US routes thing with a grain of salt, especially out in the west. I think they're laid out badly, and it's a blessing some x5 interstates don't parallel them.

But an eastern example is US 21. Even before truncation, 19, 23, 25 and 27 are longer, felt more important and more "˜major' than 21 to me. 25 got replaced by I-75 from Knoxville northward for the most part too, while I-77 wasn't even in the original interstate plans. Can't comment on 29 because it's a different trajectory than the other odd 2x.

For the 8x, US 81 doesn't really go through that many important places outside of Texas and parts of Kansas, and both 85 and 87 seemed more important (and got replaced by I-25 in the interstates era).

There's also US 99, which I'll say it's a bit more important to the west coast overall than 101, as it got replaced by I-5.

[Max Rockatansky]

Some of the corridors had a huge shift in importance over time.  Example; US 99 definitely became the more important long distance route over time compared to US 101.  The corridor of 99 ended up with way more people north of San Francisco and went through way better terrain. I would largely attribute those as the reasons why I-5 mostly follows what was US 99 north of Los Angeles.

[GaryV]

The US system was set up when many areas, especially in the West but also in the South, weren't as populated as they were by the time the Interstates came along. Snowbirds and retirement homes, industry moving south and west, lots of reasons. That accounts for some of the difference. 

There's also fewer numbers available for "major" Interstates, to keep away from duplicate numbers in the same area.  Particularly E-W, where I-50 and I-60 were skipped, so there's 8 possibilities instead of 10.  And for N-S, US-101 resulted in 11 "major" routes, where the Interstates had only 10, and one of them, I-45, was relegated to staying in Texas.  The Interstate system also was restricted in the number of miles.

I'm guessing a lot of traffic patterns changed over the 4 decades between the US and Interstate system launches.

[dkblake]

Thinking about I-85 vs I-81, from 1920 to 1960, Charlotte, Raleigh, and Durham all basically quadrupled in size and the Great Appalachian Valley cities probably remained about the same. I'd suspect by then the Piedmont region had supplanted the Great Appalachian Valley in terms of transport. I-81 remains one of the major non -0 and -5 Interstates though.

I take the x1 major US routes thing with a grain of salt, especially out in the west. I think they're laid out badly, and it's a blessing some x5 interstates don't parallel them.

Worth noting that I-15 mostly replaced US 91 since most of the route was in unpopulated areas, so US 91 formerly served its role as a long-haul route over a main transportation corridor.

[CtrlAltDel]

There's also fewer numbers available for "major" Interstates, to keep away from duplicate numbers in the same area.  Particularly E-W, where I-50 and I-60 were skipped, so there's 8 possibilities instead of 10.

It's admittedly not too terribly important, but there are 7 possibilities instead of 9.

[SkyPesos]

I take the x1 major US routes thing with a grain of salt, especially out in the west. I think they’re laid out badly, and it’s a blessing some x5 interstates don’t parallel them.

Worth noting that I-15 mostly replaced US 91 since most of the route was in unpopulated areas, so US 91 formerly served its role as a long-haul route over a main transportation corridor.

I-15 is a notable one. The OP mentioned I-55 with US 51 and 61. Here's a rough list of the US routes each x5 interstate parallels or replaced, with x1 US routes bolded

I-5: US 101 Mexico Border-LA, US 99 LA-Canadian Border
I-15: US 395 San Diego-Inland Empire, US 91 Inland Empire-Canadian Border
I-25: US 85 NM, US 85/87 CO, US 87 WY
I-35: US 81 Laredo to Ft Worth, US 77 Ft Worth to point south of OK-KS border, US 81 near Wichita, US 50 Emporia-Kansas City, US 69 Kansas City-Des Moines, US 65/69 Des Moines-Albert Lea, US 65 Albert Lea-Minneapolis, US 61 Minneapolis-Duluth
I-45: US 75 Galveston-Dallas
I-55: US 51 Laplace-Memphis, US 61 Memphis-St Louis, US 66 St Louis-Chicago
I-65: US 31 Mobile-Indianapolis, roughly US 52/231 Indianapolis-Gary
I-75: US 41 Miami-Chattanooga, US 11 Chattanooga-Knoxville, US 25 Knoxville-Detroit, US 10 Detroit-Bay City (includes US 23), US 27 Grayling-St Ignace (includes US 31), US 2 St Ignace-Canada Border
I-85: US 29 Montgomery-Greensboro, US 70 Greensboro-Durham, US 15 Durham-some point near NC-VA border, US 1 from that point to Petersburg
I-95: US 1 Miami-Jacksonville, US 17 Jacksonville-N of Savannah, US 15 portions in SC, US 301 near I-26 interchange-Richmond, US 1 Richmond-Baltimore, US 40 Baltimore-Wilmington, US 13 Wilmington-Philadelphia, US 1 Philadelphia-Portland (ME), US 202 Portland-Bangor, US 2 Bangor-Canada Border.

So each one except 25 and 45 parallel/replaced a portion of an x1 US route.

[NWI_Irish96]

The US highway network was a lot more dense than the interstate network, with a lot more diagonal routes, so you could have your primary routes stay truer to their directions rather than veering to hit major cities (yes, I know there are exceptions).

One example:

US 52 exists to get you between US 41 in Chicago and US 31 in Indianapolis, so you didn't need a dedicated route number for Chicago-Indy. With the much less dense interstate network, the same interstate number that followed US 31 from Alabama to Indiana could be used to angle up to Chicago.

[Konza]

Some random thoughts:

The Interstate System was laid out to connect major cities and, secondarily, military installations.  Not all of the US route network did that, and the ones that didn't were no blessed with an Interstate route parallel to them.

Also there's this thought that x1 and x5 US route are supposed to be major routes.  That really isn't true; only the x1 routes were originally supposed to be major routes.  Some of the x5 routes, like 35, 45, 65, and 95, have very few Interstate concurrencies or four lane sections.  There is no 55.

Also, the nation changed significantly in the thirty years between the US system being implemented and the Interstate system being built,  A car was much more of a necessity in 1956 than it was in 1926.  Roads were mostly paved, and the authorities knew what the major routes were.  I suspect the profession of Traffic Engineering refined itself seriously.

There are a lot of border border north south US routes in the western United States.  Even though they are generally longer and further apart than similar north-south routes east of the Mississippi, most are not four lanes and haven't merited an Interstate designation because the ares they connect are not densely populated and therefor there is not enough traffic to justify a more substantial road.

It's interesting to live in a state that is still growing at a fast rate.  In my corner of Arizona, US 80 used to enter the state at Yuma, no north at Gila Bend to the Phoenix area, head down to Tucson and to the border at Douglas, then back northeast into New Mexico.  The Yuma- Gila Bend stretch is now I-8.  From Phoenix east, I-10 follows the old I-80 route, but between 1926 and 1956 the copper mines and smelters in SE Arizona ceased being the state's economic engine.  It was not longer necessary to have a transcontinental highway pass through them, and I-10 was routed on a shorter routing further north.

Plus the system continues to evolve; even sixty years ago Phoenix and Las Vegas were not large enough, and thus did not have enough traffic between them, to merit an Interstate route.  Now they do, and the route is under development.

[SkyPesos]

Also there's this thought that x1 and x5 US route are supposed to be major routes.  That really isn't true; only the x1 routes were originally supposed to be major routes.  Some of the x5 routes, like 35, 45, 65, and 95, have very few Interstate concurrencies or four lane sections.  There is no 55.

There seems to be examples of pre-interstate era major routes from each odd digit. The x7s also have some notable major routes, including 17, 27, 67, 77 and 87. The x9s have 9, 19, 29, 69 and 99. 59 doesn't really seem that major outside of Texas, that's why I exclude it. Same with 89 formerly for Arizona. The x3s seem to have less than the others, as I can only think of 23 and maybe 63. 83 is very long, but it's still 2 lanes most of the way and doesn't parallel any interstates. Same with 93, outside of Phoenix-Vegas.

[roadman65]

What is more interesting to me is the fact that US 42 (an even numbered route) was replaced with  an odd numbered interstate.  And US 67 from Little Rock to Dallas (another N-S route) was replaced with even numbered I-30.

Then you have part of US 41 between Chatannooga, and Nashville get replaced with an even numbered route, I-24.

Also to mention that I-95 parts ways with US 1 from Jacksonville to Richmond instead of being routed through Columbia and Raleigh .   I-89 in NH-VT also follows US 4 in NH, but in VT it follows VT 16, US 2, and US 7.

I guess its all to the fact that demands of people have changed, but do not forget that US 21 was completely replaced (as one mentioned here already) and even though from Columbia to Hunting Island in SC is independent of a freeway, it still has both I-26 and I-95 cutting the distance from Columbia to SE SC as discussed in a thread here that wondered why I-77 does not go all the way to Savannah, GA following US 21.

[dkblake]

I take the x1 major US routes thing with a grain of salt, especially out in the west. I think they're laid out badly, and it's a blessing some x5 interstates don't parallel them.

Worth noting that I-15 mostly replaced US 91 since most of the route was in unpopulated areas, so US 91 formerly served its role as a long-haul route over a main transportation corridor.

I-15 is a notable one. The OP mentioned I-55 with US 51 and 61. Here's a rough list of the US routes each x5 interstate parallels or replaced, with x1 US routes bolded

I-5: US 101 Mexico Border-LA, US 99 LA-Canadian Border
I-15: US 395 San Diego-Inland Empire, US 91 Inland Empire-Canadian Border
I-25: US 85 NM, US 85/87 CO, US 87 WY
I-35: US 81 Laredo to Ft Worth, US 77 Ft Worth to point south of OK-KS border, US 81 near Wichita, US 50 Emporia-Kansas City, US 69 Kansas City-Des Moines, US 65/69 Des Moines-Albert Lea, US 65 Albert Lea-Minneapolis, US 61 Minneapolis-Duluth
I-45: US 75 Galveston-Dallas
I-55: US 51 Laplace-Memphis, US 61 Memphis-St Louis, US 66 St Louis-Chicago
I-65: US 31 Mobile-Indianapolis, roughly US 52/231 Indianapolis-Gary
I-75: US 41 Miami-Chattanooga, US 11 Chattanooga-Knoxville, US 25 Knoxville-Detroit, US 10 Detroit-Bay City (includes US 23), US 27 Grayling-St Ignace (includes US 31), US 2 St Ignace-Canada Border
I-85: US 29 Montgomery-Greensboro, US 70 Greensboro-Durham, US 15 Durham-some point near NC-VA border, US 1 from that point to Petersburg
I-95: US 1 Miami-Jacksonville, US 17 Jacksonville-N of Savannah, US 15 portions in SC, US 301 near I-26 interchange-Richmond, US 1 Richmond-Baltimore, US 40 Baltimore-Wilmington, US 13 Wilmington-Philadelphia, US 1 Philadelphia-Portland (ME), US 202 Portland-Bangor, US 2 Bangor-Canada Border.

So each one except 25 and 45 parallel/replaced a portion of an x1 US route.

Ah- I think I understand your point, that the overall layout of the x1s seem bunched up too far east. I'd bet that in 1926, though, the N-S routes lined up relatively well with the distribution of population in the US.

I checked Wikipedia for mean center of population of the US in 1920 and 1930. It moves during that time from near Spencer, IN to near Linton, IN. Those are both along US 231, so let's roughly assume the mean center of population is somewhere around Point Commerce. That spot is about 20-25 miles east of the US 41 corridor and 30 miles north of the US 50 corridor. So the layout of the US routes for x0 routes is roughly balanced north and south. The x1 routes, though, are actually imbalanced more toward the western half of the US population at the time than the eastern half.

[hbelkins]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense. And the existence of I-30 and I-45 that go against the nature of the "major cross-country" status that the x0 and x5 Interstates were supposed to have.

[hotdogPi]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

[Evan_Th]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

Give them imaginary numbers.  US i , US 5i, US 11i...

[Big John]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

Give them imaginary numbers.  US i , US 5i, US 11i...

Then on Interstate highways: Ii, I5i, I11i...

[JayhawkCO]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

Give them imaginary numbers.  US i , US 5i, US 11i...

Then on Interstate highways: Ii, I5i, I11i...

See, now that's just confusing.  It would clearly be easier to have I-√-1.

Chris

[kenarmy]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

How much of the route has to be "in-grid" then?? US 11 is out of grid for most of its route, but it fits perfectly by the time it reaches its northern terminus.

[SkyPesos]

Of course, you have the diagonal nature of US 11 and US 41 that never really made a lot of sense.

Long diagonals are going to violate the grid no matter what number you give them.

How much of the route has to be "in-grid" then?? US 11 is out of grid for most of its route, but it fits perfectly by the time it reaches its northern terminus.

Assuming the "grid" have the straight N-S and E-W routes as its base (excluding obvious oddities like I-73), for me, as long as there's a part of a diagonal route that's in the grid, it's fine. Though I know there's people in fictional that have much stricter requirements, like renumbering I-69 from Indy to Evansville as I-61 or I-63 because it's west of I-65, or swapping I-75 and I-85 south of Atlanta.

With that in mind, I-95 is pretty much only in its proper grid placement north of Boston. In relative to the straight N-S interstates, I-95 goes west of...
I-91 at New Haven
I-87 at NYC
I-83 at Baltimore
I-77 at Savannah, if you draw a straight vertical line from I-77's southern terminus down.
So in FL, I-95 is pretty much in the grid position of where I-77 is.

[skluth]

I think the main difference is the US grid was originally laid out over many existing roads while most of the interstate system was designed from scratch. Many states already had a system in place when the US grid was developed. The US route system also incorporated privately developed highways like the Lincoln Highway (US 30), National Road (US 40), and Dixie Highway (US 25, US 41, and others). The US grid overlaid much of what was already either in place or planned by the states. The interstate system did incorporate a few preexisting toll roads like the Oklahoma Turnpike and Indiana Toll Road along with some of the early LA freeway system, but its grid was mostly designed and built from scratch.

The interstate system has also undergone from much less modification than the US system, even excluding cases where an interstate subsumes a US route. It's far less common for an interstate being rerouted compared to how often US routes are for a number of obvious reasons; cost, stricter requirements for interstates, better system design at beginning, age of system, local interest, etc.

Despite "Defense" being in the name, the planning for interstates became more about connecting cities than bases even before the first planned interstate pavement was laid. Where interstate routes weren't planned, many states built freeways anyway (CA 99 in the Central Valley, CT 2) to connect those cities not connected by interstate. Sometimes, those highways were later added to the interstate system (WI 15 became part of I-43 and NY 17 is now mostly I 86).

[GaryV]

If we could use fractions, I-99 could have been I-79 1/2 instead.  Then it would fit the grid.

Could also use halves or quarters to get rid of east and west versions of the same number.

[CtrlAltDel]

See, now that's just confusing.  It would clearly be easier to have I-√-1.

That won't work since I − √− 1 = 0.

[HighwayStar]

See, now that's just confusing.  It would clearly be easier to have I-√-1.

That won't work since I − √− 1 = 0.

The fact that this thread has come to this   

[roadman65]

If we could use fractions, I-99 could have been I-79 1/2 instead.  Then it would fit the grid.

Could also use halves or quarters to get rid of east and west versions of the same number.

Heck some cities have numbered street alleys signed in fractions.  Though not feasible to highway numbers, but living in this illogical universe it is not a bad idea to assign fractions.

[JayhawkCO]

If we could use fractions, I-99 could have been I-79 1/2 instead.  Then it would fit the grid.

Could also use halves or quarters to get rid of east and west versions of the same number.

Heck some cities have numbered street alleys signed in fractions.  Though not feasible to highway numbers, but living in this illogical universe it is not a bad idea to assign fractions.

Grand Junction has ridiculous things like G¾ Street.

Chris