MUTCD Notice of Proposed Rulemaking (2020) now available

[Mdcastle]

One thing on my wish list is for shared lane left turn signals flashing yellow arrow doghouses could be allowed. That would be really nice to see!

It seems that they are sort of going to allow it as a result of "Interpretation Letter 4(09)-15 (I)", but only for approaches without through movements (see new section 4F.16 and figure 4F.15), approaches with a shared through and right turn lane (see new section 4F.09), and double left turns where there is a shared left and straight lane (see new section 4F.02). I can't find anything about permitting them in general for shared left turn lane approaches.

I find this a bit strange, as that interpretation above seems to have originated as a request to allow approaches without dedicated left turn lanes to operate with protected/permissive phasing using a 5-section signal and bi-modal green arrow/flashing yellow arrow signal face. It would seem this is the only thing the new MUTCD doesn't clearly mark out as being permitted when it comes to shared signal faces with flashing yellow arrows.

My interpretation of this:

FHWA also proposes to allow the option of displaying both the FYA and the steady yellow arrow in the same section for five-section shared left-turn/right-turn signal faces operating in protected/permissive mode in Section 4F.02 (existing Section 4D.17) Signal Indications for Left-Turn Movements—General, 4F.09 (existing Section 4E.21) Signal Indications for Right-Turn Movements—General, and Section 4F.16 (existing Section 4D.25) retitled, “Signal Indications for Approaches with Shared Left-Turn/Right-Turn Lanes and No Through Movement.

Is that allowing them for situations with only left and right turns in 4F.16 is an addition to allowing them for other scenarios in 4F.02

The signal in question for the original request was to allow a signal in Eden Prairie, MN with a dedicated left, shared left, and right to operate in split phase during peak times and permissive at other times, but MnDOT expanded it's use to any protected / permissive situation with one or more shared lanes, even undivided highway with no dedicated turn lanes. Normal practice seems to put FYA doghouses on roads with two or more lanes statewide, roads with one lane most of the state still uses three section Yield on Green, but some rural districts are putting up doghouses, getting statewide consistency on that is one of the agenda items MnDOT is looking at. 3 section permissive only signals with a FYA on the bottom are banned statewide so using those at these smaller intersections isn't an option.

[Dirt Roads]

So I actually submitted a public comment on something

I am concerned about Section 8A.13 creating provision for busway crossings to be treated with gates and flashing red lights in a manner similar to LRT or railroad crossings.

Railroad crossings are signalized the way they are because of the unique hazard to road users that they present - trains are unable to quickly stop, and it is absolutely crucial for any road users to not enter a railroad crossing when a train is approaching in order to avoid a crash that is likely to result in serious injury or death. Buses do not have this same limitation, and will have the same ability to stop in order to avoid a crash should the crossing be obstructed as they would if they were operating on a regular road and encountering an obstruction in a regular signalized intersection.

Signalizing busway crossings the way railroad crossings are signalized thus seems inadvisable for the same reason that overuse of warning signs is admonished against - the use of gates and flashing red lights overstates the hazard present, and risks desensitizing road users to them. It also makes it such that there will no longer be a form of highly distinct traffic control that uniquely alerts to the presence of a train. Road users may then fail to treat railroad crossings with the appropriate level of caution as a result.

For the sake of providing buses on a busway with priority over crossing traffic that allows them to maintain speed, I would instead suggest the appropriate way of doing this is with an ordinary traffic signal that rests in green for the cross street until an approaching bus pre-empts it. I would retain the option to supplement with an R10-7 DO NOT BLOCK INTERSECTION sign, as well the guidance that STOP or YIELD signs may be used in lieu of a signal if an engineering study deems this treatment adequate.

Woo!

I thought the operation of grade crossing signals was regulated by the FRA.

You are correct.  But the operation of BRT flashing light signals and gates would not be under the jurisdiction of the Federal Railroad Administration (FRA).

As it relates to using railroad crossing arms for BRT routes, I actually proposed as much to Pierce Transit (Tacoma, WA). As part of a new BRT system, I proposed a two-way side-alignment (rather than along each side, or in the center), but the plan was scrapped primarily because Pierce Transit wanted to run the BRT in the median. The addition of crossing arms never really came up as an issue.

I created these rather complex renderings to explain how it might work/look (click to enlarge...I made them small because it's fairly off-topic):

(I also recognize some technical issues with this, not the least being a post-mounted signal being mounted in the way of the crossing arm...just focus on the overall picture please).

Although you didn't use gates, I can see where the use of gates (in particular, pedestrian gates or crossing gates covering the sidewalk areas) would be of interest for BRT operations.  Control of pedestrians and passengers at station platforms is a particular problem, as many latecomers are prone to running in front of trains and buses.  All that being said, my concerns about the differences in system safety protocols used in the traffic signals versus railway signals gets even stronger.

All of this would be moot if certain traffic signals were required to be designed to the same level-of-safety (LOS) as grade crossing FLS&G systems. 

[fwydriver405]

I have a question regarding this part of the proposed changes:

383. (trimmed) FHWA also proposes to allow the option of displaying both the FYA and the steady yellow arrow in the same section for five-section shared left turn/right-turn signal faces operating in protected/permissive mode in Section 4F.02 (existing Section 4D.17) Signal Indications for Left-Turn Movements– General, 4F.09 (existing Section 4E.21) Signal Indications for Right-Turn Movements–General, and Section 4F.16 (existing Section 4D.25) retitled, "˜"˜Signal Indications for Approaches with Shared Left-Turn/Right-Turn Lanes and No Through Movement.'' FHWA proposes these changes based on Interim Approval 17,66 FHWA's Official Ruling No. 4(09)—15(I),67 and supporting research. FHWA also proposes revisions to various paragraphs and sections throughout the part to reflect these proposed changes.

- Does this allow 4-section bimodal faces to use the bottom section for clearance, permissive and protected indications, all at the same time? I can foresee some confusion if that was permitted...

- In this section, for a shared people/RT signal operating as FYA, can the flashing yellow arrow and circular green terminate with just a circular yellow, just like how at lagging people/RT (non-FYA) or split phase, how the green arrow and circular green terminate with just a circular yellow? Or does the steady yellow arrow have to appear along with the circular yellow, regardless if the signal is terminating from either permissive or protected indications.

[roadfro]

I have a question regarding this part of the proposed changes:

383. (trimmed) FHWA also proposes to allow the option of displaying both the FYA and the steady yellow arrow in the same section for five-section shared left turn/right-turn signal faces operating in protected/permissive mode in Section 4F.02 (existing Section 4D.17) Signal Indications for Left-Turn Movements– General, 4F.09 (existing Section 4E.21) Signal Indications for Right-Turn Movements–General, and Section 4F.16 (existing Section 4D.25) retitled, "˜"˜Signal Indications for Approaches with Shared Left-Turn/Right-Turn Lanes and No Through Movement.'' FHWA proposes these changes based on Interim Approval 17,66 FHWA's Official Ruling No. 4(09)—15(I),67 and supporting research. FHWA also proposes revisions to various paragraphs and sections throughout the part to reflect these proposed changes.

- Does this allow 4-section bimodal faces to use the bottom section for clearance, permissive and protected indications, all at the same time? I can foresee some confusion if that was permitted...

- In this section, for a shared people/RT signal operating as FYA, can the flashing yellow arrow and circular green terminate with just a circular yellow, just like how at lagging people/RT (non-FYA) or split phase, how the green arrow and circular green terminate with just a circular yellow? Or does the steady yellow arrow have to appear along with the circular yellow, regardless if the signal is terminating from either permissive or protected indications.

Regarding your first bullet, that's a non-compliant signal head–at least under the current 2009 MUTCD–if it's R-Y-G-GA/FYA (I admittedly have not gone through all of the proposed manual). Flashing yellow arrows are only supposed to be used where a dedicated turn lane exists, and circular indications are not supposed to be used over dedicated turn lanes. Circular green and FYA are redundant.

On your second bullet, I think it's a similar situation. If you've got a dedicated right turn lane, it would have an FYA and the circular green would not apply. A right turn FYA arrow permissive turn phase can be terminated with circular red (but the current MUTCD doesn't show a circular yellow in any of the figures, likely because the FYA is shown with dedicated right turn lanes so a solid yellow arrow would apply instead).

[fwydriver405]

Regarding your first bullet, that's a non-compliant signal head–at least under the current 2009 MUTCD–if it's R-Y-G-GA/FYA (I admittedly have not gone through all of the proposed manual). Flashing yellow arrows are only supposed to be used where a dedicated turn lane exists, and circular indications are not supposed to be used over dedicated turn lanes. Circular green and FYA are redundant.

No, the signal head that is shown here (and used a lot in ME, NH, MA, RI, CT and NJ) is originally R-Y-G-GA/YA. GA for the protected movement, YA for the clearance after the GA. The green ball acts as the permissive movement. Basically operates like a normal 5-section signal, except the green and yellow sections are combined into one, dual colour arrow section.

I was just asking if that MUTCD change applied to 5-section signals, where the green and yellow arrows have their own heads, if that also extended to 4-section signals with a bi-modal arrow, where the green and yellow arrows share the same head.

On the other hand, I just realised that people part of (people/RT) was autocorrected to "people"... oops! 

[stevashe]

Breaking news: As of yesterday, the comment period has been extended to May (was March). Glad they did this since there are many changes and this will give ample time for highway agencies to make comments.

I'm currently going through the proposed changes, and have found some interesting ones that I will post once I'm done. An example I can give now though is there is a proposal to allow exit numbering of intersections on expressways between interchanges, see a screenshot of the proposed figure below which gives the general idea. I'm not sure adding exit numbers for intersections is particularly useful myself, but I can't see a reason why it shouldn't be done either. What do you guys think about it?



On another note, I think someone may have snuck a Simpsons reference into one of the figures on this distance sign. 

[Ned Weasel]

I'm currently going through the proposed changes, and have found some interesting ones that I will post once I'm done. An example I can give now though is there is a proposal to allow exit numbering of intersections on expressways between interchanges, see a screenshot of the proposed figure below which gives the general idea. I'm not sure adding exit numbers for intersections is particularly useful myself, but I can't see a reason why it shouldn't be done either. What do you guys think about it?

This signage is beautiful!  I'd love to see it on US 71 in Kansas City, MO while it's still an expressway with at-grade intersections (which it probably will remain as for years to come).

[stevashe]

This signage is beautiful!  I'd love to see it on US 71 in Kansas City, MO while it's still an expressway with at-grade intersections (which it probably will remain as for years to come).

I was struggling to think of a good example where exit numbers for intersections would be useful, but you hit the nail on the head there! That would be a perfect location.

[US 89]

These would be perfect for roads like Bangerter Highway (SR 154) in Salt Lake - entirely controlled access, but with a mix of interchanges and at-grade intersections.

[TheStranger]

Some California examples of routes that could benefit from "numbered intersections" -

US 101 in its entirety, because of how much of it is freeway and how many random non-freeway segments there are

Route 99 north of Natomas

Santa Clara expressways

Route 17

Route 37

Route 60 (east of Moreno Valley)

Route 65 (Roseville-Marysville segment/former US 99E)

Route 126 east of Saticoy

[Great Lakes Roads]

I'm currently going through the proposed changes, and have found some interesting ones that I will post once I'm done. An example I can give now though is there is a proposal to allow exit numbering of intersections on expressways between interchanges, see a screenshot of the proposed figure below which gives the general idea. I'm not sure adding exit numbers for intersections is particularly useful myself, but I can't see a reason why it shouldn't be done either. What do you guys think about it?

I think that there are a few spots in Indiana that could use that system...

US 31 between Kokomo and Plymouth
US 30 between Valparaiso and Fort Wayne and beyond Fort Wayne to the Ohio state line
SR 49 between Valparaiso and Chesterton
Lloyd Expressway in Evansville
SR 25/US 24 between Lafayette and Fort Wayne

EDIT: Gotta add two more to the list: SR 37 between Fishers and Noblesville & the Muncie Bypass

[roadfro]

Regarding your first bullet, that's a non-compliant signal head–at least under the current 2009 MUTCD–if it's R-Y-G-GA/FYA (I admittedly have not gone through all of the proposed manual). Flashing yellow arrows are only supposed to be used where a dedicated turn lane exists, and circular indications are not supposed to be used over dedicated turn lanes. Circular green and FYA are redundant.

No, the signal head that is shown here (and used a lot in ME, NH, MA, RI, CT and NJ) is originally R-Y-G-GA/YA. GA for the protected movement, YA for the clearance after the GA. The green ball acts as the permissive movement. Basically operates like a normal 5-section signal, except the green and yellow sections are combined into one, dual colour arrow section.

I was just asking if that MUTCD change applied to 5-section signals, where the green and yellow arrows have their own heads, if that also extended to 4-section signals with a bi-modal arrow, where the green and yellow arrows share the same head.

On the other hand, I just realised that people part of (people/RT) was autocorrected to "people"... oops! 

Still a non-compliant head under current rules. (If it's a dedicated left turn lane and not a shared signal head, it can't have circular indications.) Something like this would be overly complicated to have GA, YA & FYA all in one signal section, and at that point it'd be better for the motoring public if a more standard signal head were installed.

[GaryV]

ISTR US 24 "Fort to Port" having exit numbers on some non-freeway sections?

[stevashe]

ISTR US 24 "Fort to Port" having exit numbers on some non-freeway sections?

It's certainly possible. A few expressways have/had exit numbers for intersections as discussed in this thread: https://www.aaroads.com/forum/index.php?topic=27425.msg2524059#msg2524059

[fwydriver405]

Regarding your first bullet, that's a non-compliant signal head—at least under the current 2009 MUTCD—if it's R-Y-G-GA/FYA (I admittedly have not gone through all of the proposed manual). Flashing yellow arrows are only supposed to be used where a dedicated turn lane exists, and circular indications are not supposed to be used over dedicated turn lanes. Circular green and FYA are redundant.

No, the signal head that is shown here (and used a lot in ME, NH, MA, RI, CT and NJ) is originally R-Y-G-GA/YA. GA for the protected movement, YA for the clearance after the GA. The green ball acts as the permissive movement. Basically operates like a normal 5-section signal, except the green and yellow sections are combined into one, dual colour arrow section.

I was just asking if that MUTCD change applied to 5-section signals, where the green and yellow arrows have their own heads, if that also extended to 4-section signals with a bi-modal arrow, where the green and yellow arrows share the same head.

On the other hand, I just realised that people part of (people/RT) was autocorrected to "people"... oops! 

Still a non-compliant head under current rules. (If it's a dedicated left turn lane and not a shared signal head, it can't have circular indications.) Something like this would be overly complicated to have GA, YA & FYA all in one signal section, and at that point it'd be better for the motoring public if a more standard signal head were installed.

I should also mention that Maine* and especially Rhode Island** like to use that 4-section bimodal signal for PPLT shared left/thru approaches, so I was wondering if FYA was going to be used for a shared lane approach, if that 4-section head would still be non-compliant, and have to be changed to inline or doghouse 5-section.

On the other hand, Figure 4F-6. Typical Position and Arrangements of Shared Signal Faces
for Protected/Permissive Mode Left Turns has been updated to include a diagram to show a non-FYA shared signal head without an exclusive left turn lane.

* Maine has a policy for new signals stating "No new protected left turns from a shared lane. This is very inefficient and does little to increase capacity or safety. Existing locations may remain."
** I have never seen a doghouse in Rhode Island as of 12/30/2020.

Edited to fix typo - wrong section intially inputted for the Maine footnote

[mass_citizen]

Didn't see it mentioned here, but the first few pages make it clear that Metric units will no longer be allowed in the Manual.

Absolutely ridiculous

Are you being facetious?

Nope. We're 50 years overdue on metrication. We're continuing to dig in our heels for frankly stupid reasons.

I'm sure if somehow the country reverses (again) and goes back to Metrication, FHWA will be able to put metric specs back into the MUTCD a lot faster than it will take the states to invest in and replace their signage (again)

[MCRoads]

I seem to have derailed this thread... Oopsies!

Anyway, back on topic:

I wish they would add a better "two lanes merge into center"  sign? Right now, the current way we sign this is just as a regular lane drop. But, I believe for situations where there is no obvious lane that ends, and both lanes merge into each other (as often happens on on ramps or curved 2-1 lane merges), we should sign it differently, to make merging easier. Hell, maybe all 2-1 merges should be signed this way. That way, it is really obvious that no lane has ROW, and to merge in turns. (Is that called zipper merging?) only issue I can see is we would need new pavement markings. Maybe we just simply use lane drop lines, and no merge arrows. Thoughts?

[jakeroot]

I wish they would add a better "two lanes merge into center"  sign? Right now, the current way we sign this is just as a regular lane drop. But, I believe for situations where there is no obvious lane that ends, and both lanes merge into each other (as often happens on on ramps or curved 2-1 lane merges), we should sign it differently, to make merging easier. Hell, maybe all 2-1 merges should be signed this way. That way, it is really obvious that no lane has ROW, and to merge in turns. (Is that called zipper merging?) only issue I can see is we would need new pavement markings. Maybe we just simply use lane drop lines, and no merge arrows. Thoughts?

I think the issue with signing for a zipper is that it's not necessarily supported by law. Signs posted by the road authority can usually override existing laws (for example, "no turn on red"), but if a crash occurs where "who had the right of way" becomes a concern, you end up with this grey area where all of these options become possible: neither driver, both drivers, whoever was in front, the one with the bigger vehicle, etc. But of course, none of those things are written in stone somewhere, so to avoid any sort of concern around vagueness, the general preference is to just end one lane and maybe post some informal "take turn" signage.

One area where zipper merges are actually great, and are supported by law (usually; WA's is RCW 47.52.026), are ramp meters. WSDOT just installed a four-lane ramp meter where all four lanes merge into a single lane. There are traditional merge arrows for when the meter is inactive, but when the meter is active, those are ignored and the right-of-way is granted by the traffic lights instead. In practice, roadway geometry dictates actual right-of-way (see this example), but when meters are active, and the car in the "ending lane" gets their green light first, you don't speed up to block them, as they received a green light first and so they have the right-of-way.

[stevashe]

I wish they would add a better "two lanes merge into center"  sign? Right now, the current way we sign this is just as a regular lane drop. But, I believe for situations where there is no obvious lane that ends, and both lanes merge into each other (as often happens on on ramps or curved 2-1 lane merges), we should sign it differently, to make merging easier. Hell, maybe all 2-1 merges should be signed this way. That way, it is really obvious that no lane has ROW, and to merge in turns. (Is that called zipper merging?) only issue I can see is we would need new pavement markings. Maybe we just simply use lane drop lines, and no merge arrows. Thoughts?

I think the issue with signing for a zipper is that it's not necessarily supported by law. Signs posted by the road authority can usually override existing laws (for example, "no turn on red"), but if a crash occurs where "who had the right of way" becomes a concern, you end up with this grey area where all of these options become possible: neither driver, both drivers, whoever was in front, the one with the bigger vehicle, etc. But of course, none of those things are written in stone somewhere, so to avoid any sort of concern around vagueness, the general preference is to just end one lane and maybe post some informal "take turn" signage.

One area where zipper merges are actually great, and are supported by law (usually; WA's is RCW 47.52.026), are ramp meters. WSDOT just installed a four-lane ramp meter where all four lanes merge into a single lane. There are traditional merge arrows for when the meter is inactive, but when the meter is active, those are ignored and the right-of-way is granted by the traffic lights instead. In practice, roadway geometry dictates actual right-of-way (see this example), but when meters are active, and the car in the "ending lane" gets their green light first, you don't speed up to block them, as they received a green light first and so they have the right-of-way.

Actually, they are proposing to add just this! See new signs W4-8 (symbol version) and W9-4 (text version) in the figure below:


(This figure also shows the new signs for denoting which lanes are going in each direction on a 3 lane road, which is a great addition in my opinion.)

Text from the new proposed section for these signs:

Support:

The LANES MERGE (W9-4) and Single-Lane Transition (W4-8) signs are used to warn of the reduction of two lanes to one in the same direction of travel (See Figure 2C-12).

Guidance:

The Lanes Merge (W9-4) sign should be used warn that the traffic lane is merging with the adjacent lane and a merging maneuver will be required for each lane. The W9-4 sign should be installed in accordance with Table 2C-3.

The Single-Lane Transition (W4-8) sign should be used to indicate the approximate location of the start of the lane taper.

Looks like they're allowing use of this at any location where 2 lanes merge into one, and honestly I'd agree with this. In a lot of cases it should be responsibility of drivers in both lanes to ensure the merge happens safely.

[MCRoads]

I wish they would add a better "two lanes merge into center"  sign? Right now, the current way we sign this is just as a regular lane drop. But, I believe for situations where there is no obvious lane that ends, and both lanes merge into each other (as often happens on on ramps or curved 2-1 lane merges), we should sign it differently, to make merging easier. Hell, maybe all 2-1 merges should be signed this way. That way, it is really obvious that no lane has ROW, and to merge in turns. (Is that called zipper merging?) only issue I can see is we would need new pavement markings. Maybe we just simply use lane drop lines, and no merge arrows. Thoughts?

I think the issue with signing for a zipper is that it's not necessarily supported by law. Signs posted by the road authority can usually override existing laws (for example, "no turn on red"), but if a crash occurs where "who had the right of way" becomes a concern, you end up with this grey area where all of these options become possible: neither driver, both drivers, whoever was in front, the one with the bigger vehicle, etc. But of course, none of those things are written in stone somewhere, so to avoid any sort of concern around vagueness, the general preference is to just end one lane and maybe post some informal "take turn" signage.

One area where zipper merges are actually great, and are supported by law (usually; WA's is RCW 47.52.026), are ramp meters. WSDOT just installed a four-lane ramp meter where all four lanes merge into a single lane. There are traditional merge arrows for when the meter is inactive, but when the meter is active, those are ignored and the right-of-way is granted by the traffic lights instead. In practice, roadway geometry dictates actual right-of-way (see this example), but when meters are active, and the car in the "ending lane" gets their green light first, you don't speed up to block them, as they received a green light first and so they have the right-of-way.

Actually, they are proposing to add just this! See new signs W4-8 (symbol version) and W9-4 (text version) in the figure below:


(This figure also shows the new signs for denoting which lanes are going in each direction on a 3 lane road, which is a great addition in my opinion.)

Text from the new proposed section for these signs:

Support:

The LANES MERGE (W9-4) and Single-Lane Transition (W4-8) signs are used to warn of the reduction of two lanes to one in the same direction of travel (See Figure 2C-12).

Guidance:

The Lanes Merge (W9-4) sign should be used warn that the traffic lane is merging with the adjacent lane and a merging maneuver will be required for each lane. The W9-4 sign should be installed in accordance with Table 2C-3.

The Single-Lane Transition (W4-8) sign should be used to indicate the approximate location of the start of the lane taper.

Looks like they're allowing use of this at any location where 2 lanes merge into one, and honestly I'd agree with this. In a lot of cases it should be responsibility of drivers in both lanes to ensure the merge happens safely.

... well then, I guess I'm getting my wish!

[Ned Weasel]

Actually, they are proposing to add just this! See new signs W4-8 (symbol version) and W9-4 (text version) in the figure below:

Two interesting things I've noticed!

(1) The W4-8 sign has been used IRL before, although perhaps without the dashed line (similar to W4-2, which used to not have the dashed line).  I know I saw it on Long Island, and I think it was at this interchange: https://goo.gl/maps/QbYzd2Cja1Ntuw3h7 , on the ramp from Loop Parkway to NB Meadowbrook Parkway, but if that's the case, then they re-signed and re-striped it to show the right lane ending instead.

(2) NJDOT once again made it into the MUTCD!  NJDOT has been using W9-7 with the word "FOR" for forever, but other states tended to omit the preposition.  For a previous example of NJDOT and its preposition use making it into the MUTCD, see R3-23 ("ALL TURNS FROM RIGHT LANE").

[jemacedo9]

Actually, they are proposing to add just this! See new signs W4-8 (symbol version) and W9-4 (text version) in the figure below:

(1) The W4-8 sign has been used IRL before, although perhaps without the dashed line (similar to W4-2, which used to not have the dashed line).  I know I saw it on Long Island, and I think it was at this interchange: https://goo.gl/maps/QbYzd2Cja1Ntuw3h7 , on the ramp from Loop Parkway to NB Meadowbrook Parkway, but if that's the case, then they re-signed and re-striped it to show the right lane ending instead.

That is a NY-specific sign (without the dashed line)...it is fairly common in the Rochester area.  It looks like the MUTCD is proposing to adopt it nationwide.

[JoePCool14]

(2) NJDOT once again made it into the MUTCD!  NJDOT has been using W9-7 with the word "FOR" for forever, but other states tended to omit the preposition.  For a previous example of NJDOT and its preposition use making it into the MUTCD, see R3-23 ("ALL TURNS FROM RIGHT LANE").

I don't really like the added "for". It makes the sign needlessly larger. I think "Right Lane Must Exit" would be better. It keeps the sign at a nice four words and is grammatically correct.

Kind of like this, without the added assembly on the right.

[jakeroot]

Yeah, 'for exit only' makes no sense to me.

'Must exit' seems much easier to comprehend.

[Ned Weasel]

Yeah, 'for exit only' makes no sense to me.

'Must exit' seems much easier to comprehend.

It felt a bit awkward to me the first time I saw it, which might have been around 15 years ago.  But it didn't take any mental gymnastics to make sense of it.  Frankly, I can understand why they use that wording.  "Right Lane Exit Only" could be misinterpreted as meaning the exit is only from the right and not from the left, rather than the intended meaning which we all know as, the right-hand lane is being dropped from the mainline at the exit.  "Right Lane for Exit Only" removes that ambiguity without quite making it a complete sentence (the most direct complete sentence translation would be "The right lane is for exiting only" or "The right lane is for the exit only").  As for "Right Lane Must Exit," while I can't claim to read the minds of the MUTCD gods, I suspect they don't want the word "must" on anything that isn't regulatory signage, and they in fact have a separate, black-on-white regulatory sign reading "RIGHT LANE MUST EXIT" (R3-33).