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"Diverging diamond" doesn't help make a walkable corridor

An almost-finished plan for the Greenbelt Metro and MD-193 area aims to create pedestrian-friendly urban nodes in northern Prince George's. But the county has decided to push a pedestrian and bike-unfriendly interchange in the middle of the corridor.

Diverging diamond in Springfield, MO. Photo by MoDOT on Flickr.

Prince George's planners recently held their final meeting on the Sector Plan in Greenbelt. It caps months of hard work and civic engagement. But in a baffling move, the department chose this meeting to bring up the idea of transforming the Greenbelt Road/Kenilworth Avenue interchange into an even more anti-pedestrian environment by converting it to a "diverging diamond."

None of the planners, and especially not the traffic engineer leading this part of the discussion, saw any conflict between turning one section of Greenbelt Road into a micro-freeway while turning the next block into a pedestrian-friendly urban district.

Diverging diamond: Faster traffic, worse for pedestrians

Diverging diamond interchanges (DDIs) are designed to move cars more efficiently by reducing the number of signal phases at interchanges and allowing cars on freeway on- and off-ramps to move freely without waiting for signals.

To accomplish this, the surface street lanes (not the freeway) cross to the opposite (left) side of the center line as they to pass through the interchange.

Diverging diamond diagram from Wikimedia.

Pedestrians have to cross to the median and walk between concrete walls, forcing them to cross half of the through lanes at each side of the bridge. In the case of the Greenbelt Road/Kenilworth Avenue interchange, that would mean crossing 3 or 4 lanes (depending on the final configuration of the design) at each side of the bridge.

Alternatively, the design could accommodate pedestrians the outside of the roadway, but then they must cross the free-flowing left turn on- and off-ramps where drivers will be focusing on making the turn fast rather than looking for people crossing.

Incompatible with walkable vision for the corridor

What's most troubling is that the planning department is actually trying to create an urban, pedestrian environment immediately west of the interchange, yet they still proposed this design which does the opposite.

Early on in the presentation, planners showed before and after renderings of their visions for a walkable urban node where Beltway Plaza is today. They showed a suburban arterial transformed into a narrowed street with wide sidewalks, street trees, pedestrian lighting, and bike lanes. At previous meetings, they talked of building a street grid, of filling in parking lots with development, and making it easier and safer to walk in these new urban nodes.

They also talked of finding ways to link the different neighborhoods of Greenbelt that have been separated from each other by the various freeways in the area. They specifically mentioned finding better ways of linking the Golden Triangle office park with the Beltway Plaza area—two neighborhoods that are currently separated from each other by Kenilworth Avenue and its interchange with Greenbelt Road.

But planners are approaching rebuilding the Kenilworth/Greenbelt with the objective of moving more cars, faster. They are not thinking about creating a pedestrian-friendly environment in that space. They are not thinking about making cyclists feel welcome on the road.

And encouraging drivers to speed up as they approach what planners hope to be a walkable node is asking for trouble.

The cure is far worse than the disease

Today, Greenbelt Road crosses above Kenilworth Avenue at an interchange built in the 1980s. Most would agree that the intersection has its problems, mostly from the way the northbound ramps are offset and the close spacing of the north- and southbound off-ramps.

Image modified from Google Maps.

But the solution the county planners propose would be far worse than the current setup, especially for pedestrians and cyclists.

It is questionable why the county wants to focus on this intersection to begin with. It's not "failing" by traffic engineer standards, and in terms of driver delay, it's not even the worst intersection in the corridor, according to a study conducted in conjunction with the Sector Plan. But of course, highway engineers like to "fix" things whether or not they're broken.

Strong Towns executive director Chuck Marohn narrated a video about a DDI in Springfield, Missouri. A traffic engineer involved in the design created the video, touting how a pedestrian can walk through the interchange, but Marohn points out how absurd it is to say that this is actually pedestrian-friendly.

Marohn notes that while a DDI provides a path for pedestrians and cyclists, it's nothing like the kind of interchange one would design if the goal from the start were to make a space friendly to people walking and biking.

Building walkable communities and complete streets has to be more than an engineer running down an accommodation checklist. If we're trying to create a neighborhood where walkability is a primary goal, then pedestrians, cyclists, and transit users have to be a top priority, not just get the leftover road space and the bare minimum listed in the design guide.

The nascent urban districts at White Flint and Tysons Corner are transforming from suburbs to more walkable spaces. And like the Beltway Plaza area, pedestrians in those areas face barriers in the form of interchanges. Prince George's can't simply get rid of their interchanges, but they don't need to make the pedestrian condition worse by recommending converting an interchange to one that's sole purpose is to move cars more quickly.

Matt Johnson has lived in the Washington area since 2007. He has a Master's in Planning from the University of Maryland and a BS in Public Policy from Georgia Tech. He lives in Greenbelt. Hes a member of the American Institute of Certified Planners. He is a contract employee of the Montgomery County Department of Transportation. His views are his own and do not represent those of his employer. 


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If "the goal from the start were to make a space friendly to people walking and biking" then how many pedestrians would realistically utilize this area vs. drivers in the most ideal scenario? Maybe 1 ped: 10,000 drivers?

I share your distrust of diverging diamonds, but I also don't know how realistic it is for the an area where four major automobile corridors converge to become a vibrant, walkable, mixed-use neighborhood on all sides. There are multiple major barriers to pedestrian/bicycle movement there. Seems like the vast majority of the ped focus is going to be to the north and west of this interchange.

by David on Apr 30, 2012 11:12 am • linkreport

At least it's not a diverging windmill.

by Jack Love on Apr 30, 2012 11:44 am • linkreport

David, you plan for the urban environment of your future, not accomodate past failures.

Or, perhaps the phrase, "If all you plan for is cars and traffic, all you'll ever get is cars and traffic. If you plan for people and places, you'll get people and places," is apt.

Put the automobile corridors on a diet, plain and simple. That area is too valuabe to be a traffic sewer in perpetuity.

by Cavan on Apr 30, 2012 11:48 am • linkreport

Put the automobile corridors on a diet, plain and simple. That area is too valuabe to be a traffic sewer in perpetuity.

Meh. No one moved to PG County because they wanted walkable neighborhoods. PG county is the "entry level" place for people who want to live out the fantasy of a single family home with a yard and a garage where they can drive to the supermarket without having to spend a lot of money.

The whole model you're supporting just isn't compatible with PG county's history and culture.

by JustMe on Apr 30, 2012 11:51 am • linkreport

Where do these alleged "engineers" get these foolish ideas? If they're being taught these concepts on a university level, perhaps it's time to examine some credentials.

by Common_Sense on Apr 30, 2012 12:03 pm • linkreport

"PG county is the "entry level" place for people who want to live out the fantasy of a single family home with a yard and a garage where they can drive to the supermarket without having to spend a lot of money."

Just a simple change and you'd be onto something...

"PG county is the "entry level" place for people who want to live out the fantasy of a walkable urban place with beautiful public spaces and convenient access to transit without having to spend a lot of money."

For the record, I don't think that Prince George's is or should be an "entry level" place for 'people.' People move around for a variety of reasons. I just changed your statement to highlight how backward-looking it is.

However, rents per square foot (both commercial and residential) in walkable urban places have been higher than in car-dependent places for five years now. That implies more demand than supply. Why wouldn't the market also demand such real estate in and around Greenbelt in Prince George's County? There's nothing about Prince George's that makes it any different from anywhere else in this regard.

JustMe, that model I'm supporting is compatible with Prince George's future.

by Cavan on Apr 30, 2012 12:11 pm • linkreport

Not sure why this would even need a redesign at all. Where would anyone walk to east of this interchange anyway? It's Greenbelt Park to the south of 193 and then a large hotel/business park to the north. This doesn't scream pedestrian traffic to me... just a waste of money. Perhaps it's not perfect but it's good enough as is.

by PG worker on Apr 30, 2012 12:36 pm • linkreport

I'm from Springfield, and have ridden the DDIs on both sides of town several times. I'm now a cyclist here in the District and will say, they work in very specific situations to move vehicles onto an interstate very efficiently.

Springfield is very very far from a walkable city, at least where the DDIs are, however the above interchange seems like a perfect candidate to try a DDI.

Springfield, and MODOT are fairly progressive with traffic circles, and trying new, innovative (at least in the States) approaches to moving people. I think this post unfairly criticizes DDIs because Springfield is not a walkable city, not the DDI itself.

by @SamuelMoore on Apr 30, 2012 12:39 pm • linkreport

There's nothing about Prince George's that makes it any different from anywhere else in this regard.

Thank goodness! I was sitting here wondering why in the heck someone would even call any county "entry level," esp one as large of PG.

Where would anyone walk to east of this interchange anyway? It's Greenbelt Park to the south of 193 and then a large hotel/business park to the north. This doesn't scream pedestrian traffic to me</>

Ditto. I just assumed there were plans underway to totally change the landscape or something. Certainly not an area where I walk...or even want to.

by HogWash on Apr 30, 2012 12:54 pm • linkreport

I don't think the DDI itself need be considered any more pedestrian unfriendly than most other interchange designs. I've seen a mix of DDI's which are horribly designed for peds, but I've also been through some which are better than cloverleafs, parclos, and many diamonds. In my opinion, the better target (which this article does cover, as well) is whether an interchange of any variety is what's most ideal for the location.

by Bossi on Apr 30, 2012 12:54 pm • linkreport

How much different is this proposal, than, say, the interchange of Rockville Pike and Montrose? Smaller scale, bigger scale, ?

Rockville Pike @ Montrose is sort of a half-diamond, and is a big wasteland of cement but at least there is (on one side of the bridge) a pedestrian sidewalk and I feel it is a slight improvement for pedestrians over the old intersection.

by BO on Apr 30, 2012 1:16 pm • linkreport

I'm with Bossi (and I think with Matt Johnson too). I don't think interchanges are appropriate anywhere on the Beltway, except with other interstates.

It's not just about pedestrians at the interchanges themselves. It's about transitioning drivers from a limited-access highway to an urban street. They need to come to a full stop, or nearly so (at a streetcorner with a traffic light, stop line, and right curb radius). That resets their instincts to drive slower and watch out for pedestrians and bikes.

That kind of redesign would also free up a lot of wasted land that SHA could sell off to pay for the rebuilding.

by Ben Ross on Apr 30, 2012 1:18 pm • linkreport

The thing seems to just operate like a cloverleaf, without needing as much space to engineer it. You still have traffic having to merge in and out in a short distance on the same side of the roadway. These things were created to reduce the amount of time spent sitting at lights trying to make left turns in a traditional diamond interchange. Considering this being a state project when it comes down to ultimate build because both Kenilworth and Greenbelt Roads are State Roads, the State will have the same standards of having to make any new construction pedestrian friendly. I'd suggest lobbying the state for a separate pedestrian bridge that parallels 193 and goes over all the interchanging mess of vehicles. It's not ideal visually, but would probably be safer on a piratical point.

by Gull on Apr 30, 2012 1:22 pm • linkreport

Oops - that was "tight curb radius."

by Ben Ross on Apr 30, 2012 1:23 pm • linkreport

I predict many deaths at what I will call "the stupidest intersection this side of Springfield, MO".

The only way to make the area walkable is deck over the whole thing. Good luck with that.

by Michael on Apr 30, 2012 1:30 pm • linkreport

I grew up in Hyattsville, in PG, and my neighborhood has always been walkable. My parents bought the house in the early 70s, and I could walk to school, church, playgrounds, sports fields, and the library.

by Paul C on Apr 30, 2012 1:54 pm • linkreport

Here's why I like this idea:

If the divergent diamond decreases the wait time to get on an exit without eating all the land a cloverleaf would, ostensibly they could reduce the number of lanes on Greenbelt Road. Does that make that area more walkable? Not really. But having been an "entry level resident" of Prince George's County for eight years, I can say that there is currently little need for a pedestrian friendly environment there. If it reduces the number of lanes on Greenbelt Road, that will help further accomplish walkability in the area where new development WILL be occurring.

Of course, there is no indication that these lanes will be removed. In a perfect world they'd just get rid of that whole mess and turn Kenilworth Avenue back into a regular suburban road with an at-grade intersection.

by Dave Murphy on Apr 30, 2012 1:56 pm • linkreport

No one moved to PG County because they wanted walkable neighborhoods.

Or to Rockville, or Silver Spring, or Arlington, or many other walkable neighborhoods. At least they didn't before the local governments made a commitment to TOD.

In a related note, it never fails to suprise when people make the circular argument "It's okay that it's a traffic sewer, because who would want to walk or bike anywhere in that area anyway? I mean, come on, it's a traffic sewer!"

by oboe on Apr 30, 2012 1:58 pm • linkreport

For the record, I moved to Laurel in 2003 because I wanted to be able to walk to amenities. These ignorant stereotypes about Prince George's County are really unnecessary.

by Dave Murphy on Apr 30, 2012 2:00 pm • linkreport

There's a lot of ridiculous and unfounded commentary here. In the beginning at 0:50, there is no sidewalk. The sidewalk wasn't removed to create the diverging diamond interchange. This would be a challenge for a pedestrian in any circumstance. At 01:07, the video doesn't portray the undulation in the sidewalk treatment. The yellow concrete pad has patterned 3D bubbles to alert vision-impaired pedestrians. Again, this would be the same conditions for a standard interchange. The commentator is also belittling the vision-impaired by calling street traffic noise "terrifying". That is very condescending. With "weeds on one side" and traffic on the other, the commentator is identifying that this, indeed, is a sidewalk.

At 02:00 the commentator is concerned about safety for pedestrians. There is a striped buffer zone for the turn lane and a pedestrian island refuge that's some 25 feet wide. The clear striping and concrete island serves as a delineator for anyone who wants to safely cross. The red decorative brick is to provide a CLEAR and DISTINCT refuge for pedestrians that contrast with the road. The fact that there are large jersey barriers to protect pedestrians in the median is completely undermined by the commentator's discussion on the lack of pedestrian safety. I am over 4 minutes into a 10 minute video, and nothing this guy is saying holds water. Everything is "terrifying" and the "insanity" of it all is too great for the commentator. He would do well in a padded cell; that would keep him safe from this world.

by rodgerthat on Apr 30, 2012 2:02 pm • linkreport

to some extent the chicken and egg thing is legit, though. And to some extent its about time horizons and time valu of money. When an area is built out, you arent going to change it from autocentric to walkable without massive redevelopment. Even in a place like RBC, that took over two decades. And RBC was relatively walkable in many ways to start with. Its not likely that the demand for walkable TOD, will be sufficient to transform not only huge areas in DC, but every inside the beltway suburb, and every suburb within one mile of the beltway on the outside, within say, the next 30 years. The payback on the intersection could be accomplished before that.

Now I don't know Greenbelt well, so maybe its attributes (metro station, historic center) COULD make it a better candidat for transformation. And its probably always worthwhile to look for low hanging walk/ped fruit. But the issue of a place being thoroughly unwalkable is not necessarily properly dismissed as simply being circular. Some places can be bootstrapped up - some (like Tysons, to some degree) are so valuable they justify massive public investment - and some places just won't be able to break out of their legacy built form in an economically efficient way.

by AWalkerInTheCity on Apr 30, 2012 2:08 pm • linkreport

The narrator is way too dramatic (RED BRICK TREATMENTS!!!111), but there are simple improvements that could have been made there to improve pedestrian conditions. A buffer from the sidewalk to the road, high-visibility crosswalk markings, raised crosswalks through slip lanes, etc. The interchange design, fundamentally, is motor-vehicle-focused. That's the question for 193 here - not whether this interchange design can be improved (it can), but whether a highway interchange on this scale should be built.

by Dave on Apr 30, 2012 2:56 pm • linkreport

For what it's worth, my wife and I walked along Greenbelt Road from our home in Old Greenbelt (just east of Kenilworth Avenue) to the Beltway Plaza mall (just west) yesterday (4/29), and we passed (or were passed) by at least four other people making the same trek. "Traffic sewer" or not, people do walk this east-west corridor.

by Joel on Apr 30, 2012 5:46 pm • linkreport

If there's room, I think we should install roundabouts at the exit ramps somewhat like this. This would reduce the roadway width(no turn lanes), make the intersection safer and provide better traffic flow. Ideally, I think we should return it to an at grade intersection preferably with a roundabout like the one at N. Hualapai way in Las Vegas, NV(google map it). Except ours would be little better designed since this one's from 1995. Both roads entering it are 3 lanes.

by Nic on Apr 30, 2012 5:56 pm • linkreport

As Joel points out, this interchange is right in between the highly walkable neighborhood of Old Greenbelt and the planned pedestrian environment west of Kenliworth Ave. No, it's not a perfect intersection to start with, and there may not be enough pedestrian traffic to justify improving walkability, but spending lots of money to actually make it worse for pedestrians just doesn't make sense.

by Katie on Apr 30, 2012 7:37 pm • linkreport

While I usually agree with Matt's contribs since they're pragmatic and reasonable, I disagree with this one. Having driven through that interchange many times on both roads the number of peds is probably negligible, which isn't surprising considering the more than half mile distance from Beltway Plaza to the office park east of MD201 with nothing in between.

I think the county should focus on making other more cohesive and densely populated areas nearby more walkable.

*btw another very rare interchange, a continuous-flow intersection, is already located Prince George's County in Accokeek at MD210 and MD228. The main difference between the two is that the Accokeek interchange is at grade.

by King Terrapin on Apr 30, 2012 10:11 pm • linkreport

If you want a walkable corridor how about working on the damn transit first then make the area walkable otherwise it is walkable from my residence to my parking space/parking garage to the stores parking lot/garage or streetparking to the store.

You can barely get around PG County on a Saturday outside of Seat Pleasant, District Heights, Oxion Hill, Hyattsville, and some parts of Sultland, Riverdale, Greenbelt. If you dont have a car you dont get around PG County on a Sunday unless you're near a Metrorail Station.

by kk on Apr 30, 2012 10:29 pm • linkreport

As a cyclist, I'd actually prefer to ride in the road through the interchange in the video. If I were to ride through the pedestrian 'Death Star' trench, I'd be at the mercy of traffic at every crosswalk. At least in the road I can take the lane and prevent conflicts that way. I think the idea that cyclists will prefer to take the pedestrian route through a DDI is half-baked at best.

by Ian Cooper on May 1, 2012 5:48 am • linkreport

The only thing pedestrian-unfriendly about it is that an environmentalist has "declared" it pedestrian unfriendly. His "reason"? The pedestrian does not walk in a straight line. And he didn't like the high parapet walls that were between the pedestrians and the vehicles (they were parts of the original bridges, and were left as they were).

The crossings to the center median are under signal control. And since there are no turns at those signals, no turning traffic crosses the pedestrian path.

Those traffic signals must be there. The diverging diamond does not work without them. The right turn ramps can also be signalized if necessary for pedestrians.

It is actually quite pedestrian friendly.

by MidiMagic on May 1, 2012 8:46 am • linkreport

Let's play a game called "spot the engineer" for this post. Ready. Go.

by thump on May 1, 2012 9:28 am • linkreport

Ooh, I'll play! I guess... MidiMagic. Only a transportation engineer could call that obscenity 'pedestrian friendly'.

Transportation engineers are fast becoming more loathed than politicians. Why do these folks get so much power when they are so obviously incompetent?

by Ian Cooper on May 1, 2012 10:37 am • linkreport

Nope! I'm a college computer science instructor.

You can't tell how pedestrian friendly something is just by looking at it.

And remember that the photos are of the very first DDI built in the US. We learn from experience.

Look at the DDI in Saint Louis, at the interchange of I-270 and Dorsett Rd. It has sidewalks across the sides of the interchange. They meander worse than the one going down the center (if meandering is a crime). And no way is provided to cross the crossroad, rather than crossing over or under the freeway. At least the original provides a way to get to any quadrant on foot.

by MidiMagic on May 1, 2012 11:49 am • linkreport

The only real question for me, when it comes to determining whether something could be called "pedestrian friendly", is could a 5-10 year old kid safely cross this intersection on their own. If you think your kid (or your elderly parent(s)/grandparent(s)) would feel safe/comfortable and accommodated using a diverging diamond interchange, than you probably don't walk a whole bunch. If you think a diverging-diamond increases the value of surrounding property, you probably still don't get it. If you think you'd like to visit this place, or find this a pleasant walk/bike to your local coffee hangout, you're kidding yourself.

by thump on May 1, 2012 12:08 pm • linkreport

First of all, a kid that young should not be crossing ANY highway alone. These are highways, not just streets. The age at which a kid is mature enough to cross a highway is usually somewhere near 12.

Second, there is another reason the New Jersey barriers are there. With traffic on the "wrong" side of the road, the low beam headlights, normally turned away from oncoming traffic, are now aimed at oncoming traffic. The barriers stop most of these headlight beams from getting in drivers' eyes.

by MidiMagic on May 1, 2012 12:18 pm • linkreport

@MidiMagic-Isn't that ageist? Why shouldn't a young child be able to walk to the store for a treat OR simply to get to school OR simply to play with friend on the OTHER side of a road? Why? Aren't you just doing Oboe's circular logic thing here...Kids shouldn't be there b/c it's a highway and children shouldn't be on highways?
Also, we're not just talking about kids..I also mentioned older folks. Are they not allowed to cross safely and comfort? In terms of walking pace, seniors and young children are fairly similar. By excluding one, you effectively exclude the other.
What you seem to be saying is "This is a place for cars and vehicular traffic." That's a perfectly valid opinion (though I disagree). However, that isn't what the sector plan is calling for. That community WANTS to be more walkable/bikable. This type of interchange doesn't accomplish those goals.

by thump on May 1, 2012 2:33 pm • linkreport

Put my occupation in my "name" so there's no dispute.

Exactly what about the DDI is worse than a standard interchange in the DC area, or more specifically the existing interchange? I think many are hung up on the term "pedestrian friendly". Few interchanges would fit under that header. However, I believe that the DDI is better and safer for pedestrians than almost any other interchange.

The pedestrian crossings are simple, shorter and all but one are under signal control. (the one not usually under signal control is crossing the right-turn from Greenbelt to the freeway ramp, the same condition as just about any other interchange.) Shorter means less time required to cross the roadway, meaning more pedestrian friendly for those walk slower (ie, kids or the elderly.)

So if I had a 5-10 year old, and he/she has been taught how to cross a road, then yes, this would be a LOT safer than what's out there now.

Is it perfect, no, but the perfect solution for pedestrian safety is probably a completely separated path/bridge with no interaction with other modes of travel, and I doubt that'll happen. (And most pedestrians wouldn't use such a facility, especially a bridge if it would require a significant grade change.)

I also disagree with the sub-headline saying vehicles will be faster. Faster in this case implies vehicles traveling at a higher speed. I would instead use the word "quicker" in that vehicles have shorter waits, however the speed for vehicles is lower through the interchange because of the design of the "criss-cross" at each ramp. (Technically, "faster" is accurate but I believe misleading.) Is reducing vehicular congestion bad for pedestrians? I don't think so.

by Traffic Engineer on May 2, 2012 9:09 am • linkreport

Again I repeat: the diverging diamond is not pedestrian unfriendly if it is done right. I would say a regular diamond interchange is much more unfriendly because drivers trying to turn through a gap in 3 lanes of oncoming traffic are unlikely to notice a pedestrian in the far left crosswalk. I remember once going to a job interview in Chicago and having to cross a cloverleaf interchange to get from my hotel (on one side) to the business (on the other side). THAT is pedestrian unfriendly.

Ageist? That's government's idea, not mine. That's how our local child welfare sees it, not me. "A child that young should be accompanied by a parent." I may not agree with it, but that is the (administrative) law here. When kids as young as you stated are found out on the street without supervision, the welfare charges the parents with child neglect. And school districts here are designed so that children do not walk to school across major highways.

Both of those are numbered highways, with six lanes of through traffic in each direction (not counting the turn lanes). And we are talking a quarter mile between the neighborhoods on opposite sides of these roads, with shopping centers and businesses in between. Are young kids going to walk that far? There are schools in the area, but one is a private school, and the other is a middle school (with kids old enough to cross safely).

When I was a kid in the age range you cited, I was allowed to cross any of the two-lane streets in the entire neighborhood, or ride my bicycle on those streets (obeying the vehicle traffic laws of course) once I had turned 8. But I was not allowed onto the highway until I was 12, and then just to cross or walk or my bike across at the crosswalk. The local store was on the other side of the highway. The schools were miles away, so I always rode a bus. If I wanted to visit a friend elsewhere, my parents drove me. This was in the early 1960s.

by MidiMagic on May 2, 2012 9:39 am • linkreport

Fixing a typo.

I meant 3 lanes of through traffic in each direction, not six. I went to Google Maps to count lanes, and came back with total lanes instead of lanes per direction. I changed the 4 to a 6 without reading the sentence again.

Any child in the range of 5 to 10 would have the help of a crossing guard when going to school.

by MidiMagic on May 2, 2012 9:47 am • linkreport

@ MidiMagic

Children 12 and younger cross similar streets every single day around this country and around the world. How is this road anymore dangerous than Wisconsin Ave, New York Ave, East West Hgwy, University BLVD, Georgia Ave, Rhode Island Ave, Central Ave, Annapolis Rd, Bladensburg RD etc.

It may be a highway but that means nothing as there are roads, streets and avenues that have the same amount of lanes and speedlimits.

by kk on May 3, 2012 8:46 pm • linkreport

"Exactly what about the DDI is worse than a standard interchange in the DC area, or more specifically the existing interchange?"

As I understand it, the first DDIs were retrofitted to existing interchanges to gain more capacity. But of course, they didn't gain more capacity than they would have if they had converted the overpassing road to a one-way and routed the other direction onto the next interchange down. Why didn't they do that? Because everyone thinks it's inconvenient to be forced into a detour.

So why would it be "pedestrian-friendly" to force pedestrians to detour to the median? If you are a good traffic engineer, you should understand that each sidewalk should be considered a separate roadway. Forcing pedestrians to cross to the other side is like detouring cars onto a different street.

I agree with MidiMagic, though, that DDIs can be made pedestrian-friendly. To do that, sidewalks should stay on the side, and cars should be made to turn as sharply as possible. There should also be some way of adjusting drivers to local street rather than freeway conditions, whether that is raised crosswalks, signals, etc can be determined by local conditions.

by Alex on May 4, 2012 12:17 pm • linkreport

I was visiting relatives in Columbus, IN recently, and noticed that they had installed pedestrian/bicycle underpasses at several interstate interchanges in order to accommodate both cars/trucks and pedestrians/bicycles.

This may be a way to keep these 'diverging diamaond' interchanges from becoming pedestrian and bicycle hostile.

by ray allen on Nov 16, 2015 2:33 pm • linkreport

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