Interstate exit violations

[golden eagle]

What I mean here is that since there is supposed to be at least one mile between interstate exits, what instances are there exits less than a mile of each other? Here in Jackson, exits 99 (Meadowbrook Road) and 100 (Northside Drive--my exit to get to my house) on I-55 are half a mile apart. Also, in the downtown area, exits 96A (Pearl Street) and 96B (High Street) are about half a mile away (if not less) from each other. 96B and 96C (Fortification Street) are about a half from each other too. However, since they all are part of exit 96, I may not count that as a violation.

Further north on I-55 into Madison County, the Natchez Trace Parkway (105A) and the Ridgeland/Old Agency Road (105B) exits are probably about a quarter of a mile apart (though I'm estimating here). Again, possibly not a violation since they share the same exit number.

[corco]

What I mean here is that since there is supposed to be at least one mile between interstate exits

Wait, what?  Who/what exactly said that- I've never heard of such a rule. There are hundreds if not thousands of exits within a mile of each other- I can't imagine that's a violation of a standard. I'm also not sure why it would be better to have 96A/96B than 96/97- if such a standard is in play, I can't imagine why suffixed exits would be advocated while entirely different numbers would be discouraged (especially if they're entirely different interchanges like your I-55 Jackson example)- that essentially implies that exits have to be either really close together or more than a mile apart, which would be an.... interesting standard. Please enlighten me.

[froggie]

General FHWA policy is for freeway interchanges to be at least 1 mile apart in urban areas and 3 miles apart in rural areas.  Of course, there are a gazillion exceptions to that, plus waivers can be granted if needed (i.e. you have two major routes intersecting the freeway at less than the desired distance).

I wouldn't exactly call it a "violation", because A) different standards at the beginning of Interstate construction, and B) waivers can and have been granted, and in the case of Interstates would have to be approved by FHWA to begin with.

[Truvelo]

When there are exits such as 96A and 96B doesn't it normally apply at cloverleafs where each off-ramp is called A and B?

As for numbering closely spaced interchanges couldn't a little cheating in the distances be allowed to free up a number. For example, there's an interchange at mile 95 and three close together at mile 97. Couldn't they be numbered 96, 97, 98 even though they aren't at their respective distances?

[TheStranger]

When there are exits such as 96A and 96B doesn't it normally apply at cloverleafs where each off-ramp is called A and B?

It can also be used to denote two exits within a mile, the example nearest to me here in Sacramento being US 50 eastbound exits 8A (59th Street) and 8B (65th Street).

Interestingly, California ONLY uses lettering for a cloverleaf (or any multi-ramp interchange) when there are seperate ramps that pull off the freeway mainline, and not when there is a collector-distributor road or one ramp that splits into two.  (i.e. Florin Road from Route 99 is Exits 293A and 293B, but Watt Avenue off of US 50 which utilizes a pair of collector/distributor lanes (one in each direction) is just Exit 11).

[agentsteel53]

on I-95 in Mass, there is a collector/distributor road that is signed "exits 22-23-24" (or is it 23-24-25? I don't remember).

reason being, that c/d road is the old alignment!

[Jim]

When there are exits such as 96A and 96B doesn't it normally apply at cloverleafs where each off-ramp is called A and B?

It can also be used to denote two exits within a mile, the example nearest to me here in Sacramento being US 50 eastbound exits 8A (59th Street) and 8B (65th Street).

Or a lot more than two.  The most extreme example I know of occurs in Kansas City.

http://maps.google.com/maps?ll=39.102624,-94.58117&spn=0.020015,0.028281&z=15

Edit: shortened URL.

[Truvelo]

The most extreme example I know of occurs in Kansas City.

Those exits on the northern section are too closely spaced. The weaving sections are just tiny and is it really necessary to have access to so many surface streets? Closing some of them would reduce the exit numbers by a few letters

[Brandon]

Interestingly, California ONLY uses lettering for a cloverleaf (or any multi-ramp interchange) when there are seperate ramps that pull off the freeway mainline, and not when there is a collector-distributor road or one ramp that splits into two.  (i.e. Florin Road from Route 99 is Exits 293A and 293B, but Watt Avenue off of US 50 which utilizes a pair of collector/distributor lanes (one in each direction) is just Exit 11).

That's fairly SOP around the US for C/D ramps on cloverleafs.  Outside of California, we've used that system for decades.  Nice to see California finally caught up with the rest of us.

What's even more interesing is the use of a single unsuffixed number for one direction with C/D lanes and the suffixes used for the direction without C/D lanes.  Example: I-55 and US-12/20/45 in Illinois.  Nbd is Exits 279 A&B while Sbd is Exit 279 due to the C/D lane.

[golden eagle]

What I mean here is that since there is supposed to be at least one mile between interstate exits

Wait, what?  Who/what exactly said that- I've never heard of such a rule. There are hundreds if not thousands of exits within a mile of each other- I can't imagine that's a violation of a standard. I'm also not sure why it would be better to have 96A/96B than 96/97- if such a standard is in play, I can't imagine why suffixed exits would be advocated while entirely different numbers would be discouraged (especially if they're entirely different interchanges like your I-55 Jackson example)- that essentially implies that exits have to be either really close together or more than a mile apart, which would be an.... interesting standard. Please enlighten me.

Mississippi, like most other states, numbers its interstate exits in accordance to mileage (like a mile marker) rather than sequential order (which very few states do now). Since the three downtown exits fall within the 96-mile range, that could explain why those exits are labeled A, B and C.  

[golden eagle]

I wouldn't exactly call it a "violation", because A) different standards at the beginning of Interstate construction, and B) waivers can and have been granted, and in the case of Interstates would have to be approved by FHWA to begin with.

I only used "violation" for lack of a better term.

[TheStranger]

That's fairly SOP around the US for C/D ramps on cloverleafs.  Outside of California, we've used that system for decades.  Nice to see California finally caught up with the rest of us.

California exit numbering practices seem extend that to ANY single-ramp exit though (not just cloverleafs with C/D), regardless of how many auxiliary ramps come out.  This results in signage like the following, where both the ramp from 5/99 to 80 east AND 80 west receive "Exit 522" designations -

https://www.aaroads.com/california/images005/i-005_sb_exit_522_02.jpg

Wouldn't this be designated as an A/B situation in other states?

[Scott5114]

In Oklahoma, here is how suffixes are handled with C/D roads:

For cloverleaves with C/D roads, one exit number is assigned (e.g. Exit 109 for Main Street in Norman). This exit number is on the gore signage where the C/D road departs from the mainline (often in a narrow vertical format). Gore signage at each of the ramps simply states the destination (e.g. "Main Street/Westbound").

For multiple interchanges connected by C/D roads but otherwise unrelated, one suffix will be assigned for each interchange (e.g. Exits 108AB in Norman). No gore signage is placed where the C/D road departs from the mainline. Exit tabs read "EXITS 108A-B". Gore signage at each of the ramps is the typical exit number setup.

[vdeane]

Wouldn't this be designated as an A/B situation in other states?

Not in New York.  I-81 has a couple of exceptions, but one is the result of a reconfigured interchange, and the other results from separate exits being connected by a C/D road.

[Jim]

New York uses N-S and E-W suffixes (as opposed to A-B) in cases where more than one ramp serves the same "Exit".  I don't recall seeing that practice in any other state but I don't know if it's unique to New York.

New York's I-90 east of Albany has an Exit 11E and 11W, with separate ramps, for the interchange with US 9 and 20 when approaching eastbound:



but just a single ramp for Exit 11 when approaching westbound.

[Brandon]

That's fairly SOP around the US for C/D ramps on cloverleafs.  Outside of California, we've used that system for decades.  Nice to see California finally caught up with the rest of us.

California exit numbering practices seem extend that to ANY single-ramp exit though (not just cloverleafs with C/D), regardless of how many auxiliary ramps come out.  This results in signage like the following, where both the ramp from 5/99 to 80 east AND 80 west receive "Exit 522" designations -

https://www.aaroads.com/california/images005/i-005_sb_exit_522_02.jpg

Wouldn't this be designated as an A/B situation in other states?

Nope.  In Illinois, if it is a single ramp leaving the mainline, it gets one exit number.  For example, the exit for I-355 from I-55 (both directions) splits into ramps for Nbd and Sbd I-355 after leaving the mainline.  Thus, it gets a single unsuffixed exit number, in this case Exit 269.

EDIT: In fact, I've no idea why California chose two signs for that exit when one would do.  And for goodness sakes, just put the tab above the sign.  It looks better.

[roadfro]

What I mean here is that since there is supposed to be at least one mile between interstate exits, what instances are there exits less than a mile of each other? ...

I think you've confused policy with guidelines. I believe it is AASHTO's A Policy on Geometric Design of Highways and Streets (aka the "Green Book") that recommends spacing of at least one mile between interchange ramps, and two miles between a system (freeway-to-freeway) interchange and other service (local) interchanges. This is obviously a guideline, as there are probably way too many examples of interchanges being closer than one mile to list--many of which likely date to before the 1-mile spacing recommendation was adopted.

However, the Green Book is generally developed with significant input from FHWA, the state DOTs, and various research agencies, so the Green Book is generally regarded as the Bible of U.S. street and highway design. Most public agencies and DOTs either adopt the Green Book as their design standard or base their own design guidelines substantially from it. In the case of closely-spaced interchanges, to deviate from the recommendations of the Green Book, engineers would likely have to document a rationale for this or otherwise design some mitigating measure to deal with weaving/merging issues (C/D roads, braided ramps, etc.).

[roadfro]

As for numbering closely spaced interchanges couldn't a little cheating in the distances be allowed to free up a number. For example, there's an interchange at mile 95 and three close together at mile 97. Couldn't they be numbered 96, 97, 98 even though they aren't at their respective distances?

Nevada DOT has done this on occasion. The best example is I-80 in Reno/Sparks. Due to the close spacing of interchanges east of Wells Avenue, the US 395 interchange and all exits in Sparks are actually numbered sequentially. The result ends up being that Exit 21 (the last exit in the urban area) is roughly three or four miles from Exit 22 (the first exit outside the urban area). Part of the reason I think this was done was to avoid labeling the major interchange at US 395 as Exit 14B, when it's clearly a separate entity from the minor service interchange of Wells Avenue (Exit 14, 14A if sequential).

California exit numbering practices seem extend that to ANY single-ramp exit though (not just cloverleafs with C/D), regardless of how many auxiliary ramps come out.  This results in signage like the following, where both the ramp from 5/99 to 80 east AND 80 west receive "Exit 522" designations -

https://www.aaroads.com/california/images005/i-005_sb_exit_522_02.jpg

Wouldn't this be designated as an A/B situation in other states?

No. Even though the south-to-east ramp crosses over I-80 on the same bridge as the mainline I-5 south lanes (separated by a curb island), both directions exit at the same exit gore (Exit 522) and split in half downstream of the exit gore.

Perhaps if this was a C/D setup in some other states, you might get a situation where the main exit is labeled Exit 522, then the ramp to I-80 West would be 522A and the ramp to I-80 East would be 522B.  For this exact ramp configuration, most other states would likely combine these two BGSs into one sign panel.

New York uses N-S and E-W suffixes (as opposed to A-B) in cases where more than one ramp serves the same "Exit".  I don't recall seeing that practice in any other state but I don't know if it's unique to New York.

As far as I can recall, this is primarily a New York thing (although it may have been used in other areas previously).

Using direction letters (N, S, E, W) as a suffix for exit numbers is prohibited in the national MUTCD.

[TheStranger]

EDIT: In fact, I've no idea why California chose two signs for that exit when one would do.  

CalTrans SOP is to replace previously-existing signage with reflective replacements that use the same exact text and sign height/width whenever possible, as opposed to installing a new sign with different formatting, new legends, and a wider area to accomodate more of the gantry space.

[florida]

The most extreme example I know of occurs in Kansas City.

Those exits on the northern section are too closely spaced. The weaving sections are just tiny and is it really necessary to have access to so many surface streets? Closing some of them would reduce the exit numbers by a few letters

It's very unique. Closing some of them would be a sad day


I-4, in the Deltona-Orange City-DeBary triad, used to have Exits 53-CA and 53-CB at present-day Exit 111. Old Exit 53 (now Exit 108) was for Dirksen Drive and the only other road, Enterprise Road, crosses I-4 at an angle with not much room for an interchange on the east side. I could understand it if they were 53-AA and 53-AB, but I don't understand the logic of using the "C" unless it was to differentiate the "A" and "B" "sub-suffixes".

[mightyace]

Tennessee is inconsistent in signing collector-distributor roads and ramps.  I need go no further than the I-65/TN 840 junction for an example.

Here's the sign for TN 840 on I-65 north:

This is a "distributor" road only but the ramps to TN 840 East (A) and West (B) are clearly labeled on the mainline.

The empty space will, most likely, be filled with Memphis when 840 is finished.

Here's the sign at the gore point: (It could be called the Al Gore [Sr.] point as I-65 in Tennessee is the The Albert Arnold Gore Sr. Memorial Highway.  )

Here the I-65 exit is listed as Exit 31 without any suffixes.

However when you have to pick north or south:

Exits 31A and 31B appear.

All photos here appear in the TN Road Photos collection on Flickr. (http://www.flickr.com/photos/mightyace/sets/72157615512333764/)

[roadfro]

^ It doesn't make sense to me for the ramp to have separate suffixes when it simply divides in half.  It'd be one thing if it was a C/D road, where one ramp branches followed by another with other movements merging in between them. In essence, there is no Exit 31B in this example.

[corco]

Mississippi, like most other states, numbers its interstate exits in accordance to mileage (like a mile marker) rather than sequential order (which very few states do now). Since the three downtown exits fall within the 96-mile range, that could explain why those exits are labeled A, B and C.  

I get that part- what I don't get is why three completely different interchanges that are so close to each other they have to share the same exit number wouldn't count as "violations" but two interchanges just far enough apart that they have different exit numbers would. If there's ramps shared, I get why that works, but in the Jackson example 96 A, B, and C don't share a single ramp- they're completely independent of each other.

[vdeane]

New York uses N-S and E-W suffixes (as opposed to A-B) in cases where more than one ramp serves the same "Exit".  I don't recall seeing that practice in any other state but I don't know if it's unique to New York.

Not all of New York.  The Rochester and Buffalo areas use A/B.

[Troubleshooter]

I-90 & I-94 in Chicago has a ramp for every street in the downtown area, with left entrances and right exits just north of I-290.