The "Father of the DDI" Defends Left Hand Exit and Entrance Ramps

[kernals12]

Left hand ramps are frowned upon by highway engineers due to the safety problem posed by having people merge into the fast lane when they're entering the freeway, but in 2011, Gilbert Chlewicki, the man who invented the diverging diamond interchange (or rather reinvented it after France tried it and gave up on it in the 70s, decided to defend the undefendable.

Cost Savings — Left exits can easily save $50-$100 Million (or more) on a construction project for major interchanges. The reason is that left exits are more direct than right exits, which means less land is needed for the interchange as a whole and expensive fly-over ramps with long bridges can be avoided.

Lane Balancing — When all ramps are on the right side of the highway, it requires a heavier load of traffic in the right lanes of the highway. When an interchange has a high volume of traffic using the ramps, this can cause delays on the entire highway due to the heavy weaving needed that slows traffic down. When left ramps are introduced, traffic can be more evenly distributed among all the lanes of the highway, which can improve overall traffic flow.

Thru Lane Continuity — When there is a high volume of traffic for the off-ramps, it is often necessary to peel one of the thru lanes of the highway into the ramp. For instance, suppose 1,500 vehicles/hr want to go onto a westbound ramp and another 1,500 vehicles/hr want to go onto an eastbound ramp. If, as suggested by the AASHTO Policy on Geometric Design (the Green Book), we have one ramp on the right for both movements, followed by a split on that ramp, we would require at least two lanes of traffic on the initial off-ramp. For smooth traffic flow, this would likely require one of the thru lanes on the highway to be peeled off to the ramp. If a left exit were introduced for one of those movements, no consideration would be needed to convert a highway thru lane into a ramp lane.

Elimination of Weaving Concerns within the Interchange — The simplest interchange form which requires no intersections within the interchange is the full cloverleaf. But the cloverleaf is an inefficient and often unsafe form due to the short weaving concerns. Many cloverleaf interchanges are being converted into other interchange forms to eliminate the weaving. Left side ramps are very efficient at avoiding weaving issues within the interchange.

And they still build them today. They used them on the Tip O'Neill tunnel because it allows entrances and exits to be neatly tucked into the Rose Kennedy Greenway, whereas if they had used right hand ramps, they would've had to demolish several buildings to make space for the portals. And in Santa Clara County, they want to grade separate the Lawrence Expressway using left hand exit and entrance ramps. And with driverless cars, the safety problems could disappear, so maybe this will become much more common.

[JoePCool14]

I think left exits and entrances can be okay. Like any interchange design, it just needs to be designed and built properly. If you don't have enough room for acceleration/deceleration, the ramp will be sketchy no matter what side of the highway it's on. Many of us know how bad EB I-69 to SB I-75 in Michigan is because of a complete lack of acceleration space despite being on the right.

The one thing I think we can almost all agree on is the terrible concept that full cloverleaves are on anything beyond the most minor of traffic volumes. I'd rather have left exits than cloverleaves, as long as the design is decent and there's ample warning the exit is on the left.

DDIs are solid interchanges IMO.

[1995hoo]

I recognize he's not talking solely about DDIs and is instead talking about left-side ramps in general, but in the DDI context I would argue that left-side ramps in a standard DDI are a different circumstance anyway because the "fast lane" (more properly the "passing lane") is not an issue, as the road with the left-side ramps is, by definition, not going to be a freeway due to there being traffic lights at either end of the DDI where the directions cross over. If there were to be a modified DDI that used grade-separated crossovers to eliminate the two traffic lights, that might be a different scenario.

I didn't click your link (don't have time), but one problem I've noted with left-side ramps in general-purpose lanes (as opposed to managed lanes, which generally carry fewer vehicles) is that it exacerbates the incredibly annoying tendency some people have to camp in the left lane. "I want to exit on the left six miles down the road, so I'm going to get over to the left lane immediately." Ugh.

[kernals12]

I think left exits and entrances can be okay. Like any interchange design, it just needs to be designed and built properly. If you don't have enough room for acceleration/deceleration, the ramp will be sketchy no matter what side of the highway it's on. Many of us know how bad EB I-69 to SB I-75 in Michigan is because of a complete lack of acceleration space despite being on the right.

The one thing I think we can almost all agree on is the terrible concept that full cloverleaves are on anything beyond the most minor of traffic volumes. I'd rather have left exits than cloverleaves, as long as the design is decent and there's ample warning the exit is on the left.

DDIs are solid interchanges IMO.

Don't get me started on cloverleaves. I have to use the i-93/i-95 one in Reading to get to work. It's not so bad now but a year ago, before the pandemic it was a nightmare.

[JoePCool14]

I recognize he's not talking solely about DDIs and is instead talking about left-side ramps in general, but in the DDI context I would argue that left-side ramps in a standard DDI are a different circumstance anyway because the "fast lane" (more properly the "passing lane") is not an issue, as the road with the left-side ramps is, by definition, not going to be a freeway due to there being traffic lights at either end of the DDI where the directions cross over. If there were to be a modified DDI that used grade-separated crossovers to eliminate the two traffic lights, that might be a different scenario.

I didn't click your link (don't have time), but one problem I've noted with left-side ramps in general-purpose lanes (as opposed to managed lanes, which generally carry fewer vehicles) is that it exacerbates the incredibly annoying tendency some people have to camp in the left lane. "I want to exit on the left six miles down the road, so I'm going to get over to the left lane immediately." Ugh.

I can definitely see that being a problem. If you don't give enough advanced noticed, it's confusing. But if you give too much advanced notice, people camp in the left lane. As with all things there's never a one size fits all solution.

I will say that in urban and suburban areas, for instance the Tri-State in Chicagoland, there's so much traffic that all lanes are filled with cars. Yes, the slower traffic generally stays on the right, but it's rare to get an open left lane. In that instance, a left exit with campers wouldn't make things much worse than they already are.

[webny99]

I will say that in urban and suburban areas, for instance the Tri-State in Chicagoland, there's so much traffic that all lanes are filled with cars. Yes, the slower traffic generally stays on the right, but it's rare to get an open left lane. In that instance, a left exit with campers wouldn't make things much worse than they already are.

Here's the catch, though: You need people to keep right even when traffic is light, that way things keep moving for as long as possible and don't jam up as quickly. Sure, it might be so busy that it doesn't matter at 8 AM, but when people don't care about keeping right at 5 AM, it starts to hold up the faster traffic and slow the whole system down as it gets busier.

[kernals12]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

[hotdogPi]

I think left exits and entrances can be okay. Like any interchange design, it just needs to be designed and built properly. If you don't have enough room for acceleration/deceleration, the ramp will be sketchy no matter what side of the highway it's on. Many of us know how bad EB I-69 to SB I-75 in Michigan is because of a complete lack of acceleration space despite being on the right.

The one thing I think we can almost all agree on is the terrible concept that full cloverleaves are on anything beyond the most minor of traffic volumes. I'd rather have left exits than cloverleaves, as long as the design is decent and there's ample warning the exit is on the left.

DDIs are solid interchanges IMO.

Don't get me started on cloverleaves. I have to use the i-93/i-95 one in Reading to get to work. It's not so bad now but a year ago, before the pandemic it was a nightmare.

That's because of the lane drop at Route 28, not because of the cloverleaf.

[interstatefan990]

Don't left entrances almost always have yield signs posted on them?

[JoePCool14]

Don't left entrances almost always have yield signs posted on them?

Not the ones that I've seen.

[1995hoo]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

This makes it sound like you're advocating for driving on the left, like in the UK. That would be fine with me, aside from the obvious practical problems of having to replace all the road signs and traffic lights and then having over 99 percent of the cars on the road having the steering wheels on the wrong side. I've driven on the left in the UK and I never had any issues with it.

But from the summary of the article you posted, I construed the writer's point as being that there should not necessarily be a preference for having exit ramps on either particular side, whether the right or the left, and that instead they should just go where it seems most appropriate in a given situation. From your comment quoted above, it sounds like you're suggesting left-side ramps should be the standard and right-side ramps should be the exception. I don't think that makes any sense for a variety of reasons. One obvious one is the outside ramps in a cloverleaf (i.e., the "non-loop" ramps)–it makes complete sense to have those exactly where they are on the right side because that is the shortest distance for that maneuver and it requires the least amount of construction. Even if I accept the idea that left-side ramps may sometimes be desirable or preferable–and I think I can accept that premise in certain road configurations–that doesn't mean right-side ramps should be the "exception," rather than the norm, in countries where you drive on the right.

[rickmastfan67]

Don't left entrances almost always have yield signs posted on them?

Not the ones that I've seen.

Only 2 'left' entrances I know of in the Pgh area that have a yield sign for them: https://goo.gl/maps/mXiPXKSgzPsS3r2s9 https://goo.gl/maps/GsseTe581iGmSii27

[kernals12]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

This makes it sound like you're advocating for driving on the left, like in the UK. That would be fine with me, aside from the obvious practical problems of having to replace all the road signs and traffic lights and then having over 99 percent of the cars on the road having the steering wheels on the wrong side. I've driven on the left in the UK and I never had any issues with it.

But from the summary of the article you posted, I construed the writer's point as being that there should not necessarily be a preference for having exit ramps on either particular side, whether the right or the left, and that instead they should just go where it seems most appropriate in a given situation. From your comment quoted above, it sounds like you're suggesting left-side ramps should be the standard and right-side ramps should be the exception. I don't think that makes any sense for a variety of reasons. One obvious one is the outside ramps in a cloverleaf (i.e., the "non-loop" ramps)–it makes complete sense to have those exactly where they are on the right side because that is the shortest distance for that maneuver and it requires the least amount of construction. Even if I accept the idea that left-side ramps may sometimes be desirable or preferable–and I think I can accept that premise in certain road configurations–that doesn't mean right-side ramps should be the "exception," rather than the norm, in countries where you drive on the right.

But given the context, you should clearly be able to tell that I'm not advocating for driving on the left. That ability to draw on previous knowledge to figure out somewhat ambiguous situations is a key part of human intellect, one which AI engineers envy.

[hotdogPi]

If you enter on the left and exit on the right at the next exit, or the inverse, you'll have to change several lanes quickly.

[interstatefan990]

Don't left entrances almost always have yield signs posted on them?

Not the ones that I've seen.

Only 2 'left' entrances I know of in the Pgh area that have a yield sign for them: https://goo.gl/maps/mXiPXKSgzPsS3r2s9 https://goo.gl/maps/GsseTe581iGmSii27

If you look at I-244 in Tulsa, OK, you'll see several of its left entrances have yield signs.

https://goo.gl/maps/6vy5A7D3YHTxLxHn8

https://goo.gl/maps/BJo7CrNDCjXnsFYx8

https://goo.gl/maps/4BBqZEAe3n32m35Q7

[1995hoo]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

This makes it sound like you're advocating for driving on the left, like in the UK. That would be fine with me, aside from the obvious practical problems of having to replace all the road signs and traffic lights and then having over 99 percent of the cars on the road having the steering wheels on the wrong side. I've driven on the left in the UK and I never had any issues with it.

But from the summary of the article you posted, I construed the writer's point as being that there should not necessarily be a preference for having exit ramps on either particular side, whether the right or the left, and that instead they should just go where it seems most appropriate in a given situation. From your comment quoted above, it sounds like you're suggesting left-side ramps should be the standard and right-side ramps should be the exception. I don't think that makes any sense for a variety of reasons. One obvious one is the outside ramps in a cloverleaf (i.e., the "non-loop" ramps)–it makes complete sense to have those exactly where they are on the right side because that is the shortest distance for that maneuver and it requires the least amount of construction. Even if I accept the idea that left-side ramps may sometimes be desirable or preferable–and I think I can accept that premise in certain road configurations–that doesn't mean right-side ramps should be the "exception," rather than the norm, in countries where you drive on the right.

But given the context, you should clearly be able to tell that I'm not advocating for driving on the left. That ability to draw on previous knowledge to figure out somewhat ambiguous situations is a key part of human intellect, one which AI engineers envy.

Correct, but you did refer to the right lane becoming what you call the "fast lane" (I assume you mean to refer to the passing lane). That's a hallmark of countries where you drive on the left–the right lane is the passing lane there because the traffic patterns are essentially reversed from what they are here. As an aside, the weird California practice of referring to lanes by number would make a lot more sense in a country where you drive on the left because the lane numbers would be in the proper order going left to right. But I digress. That particular part of your comment stood out to me because it seems you are suggesting there should be some sort of complete reversal in traffic patterns, and I found that to be rather preposterous on the whole unless you were to advocate completely switching sides of the road.

If you don't mean that, then I will reiterate that one problem with not having consistency as to which side the exits and entrances are on would mean that there would really be no predictability as to what should be the "passing lane" (or, as some people erroneously call it, the "fast lane"). You'd have people merging into traffic, and both accelerating and decelerating, on both sides. On a highway with three lanes in each direction, would that mean that the center lane should be the passing lane? "Keep Right or Left Except to Pass"? On a highway with two lanes in each direction, there would be no "passing lane" (due to regular exits on both sides) and the result would invariably be the need for a lot more lane-changing, which most authorities agree is one of the things that leads to more accidents, and in turn more congestion due to the accidents, blockages, rubbernecking, etc.

Despite your condescending reply, I'm not trying to attack you (this time)–rather, I'm trying to carry your comments out to explore the consequences of what would happen if the idea were implemented.

If you enter on the left and exit on the right at the next exit, or the inverse, you'll have to change several lanes quickly.

This was a long-time problem on the Beltway in Virginia when the I-66 interchange, which is less than a mile north of the next interchange to the south and about two miles south of the next one to the north, had a left-side exit and entrance on the Inner Loop and people cut across four lanes of traffic to get to and from those ramps.

Also, despite the comment below from ari-s-drives, this was not necessarily a situation of short trips because I-66 carries a lot of longer-distance traffic and the next exit to the north is a major business district (Tysons Corner).

[ari-s-drives]

If you enter on the left and exit on the right at the next exit, or the inverse, you'll have to change several lanes quickly.

Looking at the source, the DDI guy says that this could discourage making short trips by freeway rather than surface streets.

The second issue discussed was heavy lane shifting. This is a legitimate concern when two interchanges are closely spaced. When this happens, there are still ways to mitigate the weaving concern. A barrier can be placed in the highway that prohibits dangerous weaving from one ramp to the next. Drivers who would have made that weave but are now prohibited can be suggested an alternate route or be provided with an additional left side ramp to make the movement safely.
...
Left side ramps could provide a possible benefit by discouraging very short trips on the highway that can be done with the local network. Imagine if each interchange on a highway alternated with left side ramps and right side ramps with barriers between the interchanges that would prohibit drivers to weave between adjacent interchanges. This could improve the operations of the highway significantly and make better use of our full roadway network.

[kernals12]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

This makes it sound like you're advocating for driving on the left, like in the UK. That would be fine with me, aside from the obvious practical problems of having to replace all the road signs and traffic lights and then having over 99 percent of the cars on the road having the steering wheels on the wrong side. I've driven on the left in the UK and I never had any issues with it.

But from the summary of the article you posted, I construed the writer's point as being that there should not necessarily be a preference for having exit ramps on either particular side, whether the right or the left, and that instead they should just go where it seems most appropriate in a given situation. From your comment quoted above, it sounds like you're suggesting left-side ramps should be the standard and right-side ramps should be the exception. I don't think that makes any sense for a variety of reasons. One obvious one is the outside ramps in a cloverleaf (i.e., the "non-loop" ramps)–it makes complete sense to have those exactly where they are on the right side because that is the shortest distance for that maneuver and it requires the least amount of construction. Even if I accept the idea that left-side ramps may sometimes be desirable or preferable–and I think I can accept that premise in certain road configurations–that doesn't mean right-side ramps should be the "exception," rather than the norm, in countries where you drive on the right.

But given the context, you should clearly be able to tell that I'm not advocating for driving on the left. That ability to draw on previous knowledge to figure out somewhat ambiguous situations is a key part of human intellect, one which AI engineers envy.

Correct, but you did refer to the right lane becoming what you call the "fast lane" (I assume you mean to refer to the passing lane). That's a hallmark of countries where you drive on the left–the right lane is the passing lane there because the traffic patterns are essentially reversed from what they are here. As an aside, the weird California practice of referring to lanes by number would make a lot more sense in a country where you drive on the left because the lane numbers would be in the proper order going left to right. But I digress. That particular part of your comment stood out to me because it seems you are suggesting there should be some sort of complete reversal in traffic patterns, and I found that to be rather preposterous on the whole unless you were to advocate completely switching sides of the road.

If you don't mean that, then I will reiterate that one problem with not having consistency as to which side the exits and entrances are on would mean that there would really be no predictability as to what should be the "passing lane" (or, as some people erroneously call it, the "fast lane"). You'd have people merging into traffic, and both accelerating and decelerating, on both sides. On a highway with three lanes in each direction, would that mean that the center lane should be the passing lane? "Keep Right or Left Except to Pass"? On a highway with two lanes in each direction, there would be no "passing lane" (due to regular exits on both sides) and the result would invariably be the need for a lot more lane-changing, which most authorities agree is one of the things that leads to more accidents, and in turn more congestion due to the accidents, blockages, rubbernecking, etc.

Despite your condescending reply, I'm not trying to attack you (this time)–rather, I'm trying to carry your comments out to explore the consequences of what would happen if the idea were implemented.

If you enter on the left and exit on the right at the next exit, or the inverse, you'll have to change several lanes quickly.

This was a long-time problem on the Beltway in Virginia when the I-66 interchange, which is less than a mile north of the next interchange to the south and about two miles south of the next one to the north, had a left-side exit and entrance on the Inner Loop and people cut across four lanes of traffic to get to and from those ramps.

Also, despite the comment below from ari-s-drives, this was not necessarily a situation of short trips because I-66 carries a lot of longer-distance traffic and the next exit to the north is a major business district (Tysons Corner).

I mean that if left hand ramps were the norm, then for safety reasons, the passing lane would be the right lane for the same reason that the left lane is the passing lane in our world where right hand ramps are the norm

[1995hoo]

I mean that if left hand ramps were the norm, then for safety reasons, the passing lane would be the right lane for the same reason that the left lane is the passing lane in our world where right hand ramps are the norm

But based on the article summary you posted, it seems the writer there is not advocating making left-side ramps "the norm" (a phrase I understand to mean "the ordinary practice" or "the preferred practice," i.e., the standard, such that in this case right-side ramps would be "the exception"). You included a reference to "lane balancing" in the quote box in your original post, and it refers to traffic being more "evenly distributed." That implies both left- and right-side ramps.

[hotdogPi]

There's space to put ramps on the outside of a freeway. It's much harder to put ramps on the inside.

[webny99]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

...

I mean that if left hand ramps were the norm, then for safety reasons, the passing lane would be the right lane for the same reason that the left lane is the passing lane in our world where right hand ramps are the norm

OK, so aren't you really wondering why left hand ramps were chosen in the first place, not right hand ones?

[JoePCool14]

Don't left entrances almost always have yield signs posted on them?

Not the ones that I've seen.

Only 2 'left' entrances I know of in the Pgh area that have a yield sign for them: https://goo.gl/maps/mXiPXKSgzPsS3r2s9 https://goo.gl/maps/GsseTe581iGmSii27

If you look at I-244 in Tulsa, OK, you'll see several of its left entrances have yield signs.

https://goo.gl/maps/6vy5A7D3YHTxLxHn8

https://goo.gl/maps/BJo7CrNDCjXnsFYx8

https://goo.gl/maps/4BBqZEAe3n32m35Q7

I think those have yield signs because of how short the acceleration space is. Here's a couple that I know of that do not have them.

https://goo.gl/maps/HeqcnW42XJzGQcwj8

https://goo.gl/maps/xSRSj5WLaPojzLpM8

[kernals12]

It seems to me that the problems caused by left hand ramps is solely due to them not being normal. If they were the norm then the rightmost lane would be the fast lane and the problem goes away. I wonder why they chose right hand ramps in the first place.

...

I mean that if left hand ramps were the norm, then for safety reasons, the passing lane would be the right lane for the same reason that the left lane is the passing lane in our world where right hand ramps are the norm

OK, so aren't you really wondering why left hand ramps were chosen in the first place, not right hand ones?

Yes I am. Frankly I think it's superior from the standpoint of cost and right of way requirements.

[kernals12]

There's space to put ramps on the outside of a freeway. It's much harder to put ramps on the inside.

Most freeways have wide medians

[US 89]

There's space to put ramps on the outside of a freeway. It's much harder to put ramps on the inside.

Most rural freeways have wide medians

FTFY. I cannot think of a single urban freeway in either the Salt Lake or Atlanta area (the two metros I frequent most) with big enough medians to fit new left-exit ramps in.

The "median" on most urban freeways I drive usually amounts to a tall concrete barrier.