Traffic Planners Battle the Left Turn

Diverging Diamond Springfield

A story on Talk of the Nation earlier this month piqued the interest of my inner urban planner and transportation geek.  It was about the efforts of traffic engineers to design intersections that minimize potential points of conflict and maximize the flow of vehicles.  Particularly, they talked about something called a “Diverging Diamond Interchange” or DDI.

Diamond Interchange

The classic diamond interchange is very inefficient and results in nearly two dozen points of potential conflict between cars going different directions.  Every left turn, whether to enter or exit the freeway, results in the rest of the traffic having to stop.

Michigan Left

One solution known as the “Michigan Left” (because it is used mostly in Michigan, I suppose) is a relatively dangerous solution that has drivers making a u-turn about 200 meters after the intersection, then making a right-hand turn at the intersection.  The disadvantage of these uncontrolled u-turns is that one thing drivers don’t do particularly well is judge the speed and distance of oncoming vehicles. 

87th Street

Another solution is the SPUI – Single Point Urban Interchange.  This design has all traffic coming together at a single intersection controlled by a single set of lights.  I’m familiar with this as it is the new design that was built near where I used to live in suburban Kansas City.  It is an elegant design but still has some problems, not the least of which is that the space in the middle of the intersection is very large and people can get lost.  I’ve observed this on several occasions, where cars have drifted into the wrong direction, especially at night when traffic is light and visual cues (like the headlines of oncoming cars) not so available.

If you have never had the pleasure of driving through an SPUI and are curious how they work, here’s a nice short video that shows an animation of traffic going through the intersection. 

Diamond 1

The latest innovation, which so far has only been built a few places in the United States, is something called the Diverging Diamond Interchange, or DDI.  Somewhat counterintuitively, the DDI involves the lanes of traffic switching sides on each end of the interchange.  The result is that there are only two controlled intersections and drastically fewer potential points of conflicts.  All left turns follow the natural, uninterrupted flow of traffic.  (Thanks to NPR for the graphics.)

Here is how it works:

Diamond 2

As you approach the traffic signal, the lanes of traffic curve slightly to the left, passing oncoming traffic (which waits at the light) at a 25-degree angle.

Diamond 3

As you travel through the signal, you are on the left hand side of the road.

Diamond 4

If you are turning left onto the freeway, you simply make a left-hand turn without waiting for a light.  Through traffic keeps on moving, not having to stop for cars waiting to make left-hand turns.

Diamond 5

After passing through the second intersection, traffic crosses back over to the right-hand side of the road.

Diamond 6

Exiting from the freeway to go left onto the arterial road, you merge and do not have to go through an intersection.

Diamond 7

Entering the freeway with a right hand turn is also a merge and does not require you to go through an intersection.  In all, the traffic flows much more smoothly. 

Here’s a short animation that shows the traffic flow.

There are multiple safety benefits.  Traffic keeps moving, reducing standstill time and the risk of rear-end crashes.  Right-angle crashes are eliminated for drivers turning left onto the freeway, as they no longer cross oncoming traffic.  Finally, fewer intersections and independent directions to cross those intersections means fewer collisions caused by people running lights.

All told, I’m sold on the Diverging Diamond Interchange.  Looks like a great way to improve traffic flow and reduce the risks of accidents.

One other option that is interesting is something called the Pinavia interchange, which I guess has been built in Europe.  It is an elaborate interchange that requires no intersections and is aesthetically pleasing, too.  Above is a brief animation to give you an idea of how it works.

Anyhow, that’s enough urban planning/transportation geekiness for today.  But isn’t it fantastic, the things you can learn on NPR?


Turning the other cheek

What a busy few days.  Don’t know if you saw, but my last entry asking for everyone to support the “No on Proposition 8” campaign ended up as a featured entry on the front page of Xanga.  As of Sunday evening my time, there are 2600+ views.

The good news is that about 80% of the 150+ comments are supportive.  The bad news is that the 20% who don’t agree with my position are probably not going to be swayed.

I’ve made it a point to respond to all comments, even though trying to refute the same arguments is tiring.  The ones that are easier to refute are the ones based in legal precedent: for example, people who don’t like the “activist” judges who “overturned democracy” – forgetting that 3 of those 5 judges are Republicans, appointed by Republican governors.

The more difficult ones to refute are the “I don’t like gay marriage because my god says so” arguments.  If you are convinced that you are righteous, what can I say to change your mind.  I’ll just have to wait until their day of judgement when, standing on heaven’s doorstep, God asks them what part of Jesus’ teachings they didn’t understand.  Was it the “love your neighbor as yourself” part?  The “worry about the log in your own eye before you worry about the splinter in someone else’s eye” part?  Maybe the “let he who is without sin cast the first stone” teaching? 

Personally, I take great comfort in Matthew 5:11:

“Blessed are ye, when men shall revile you, and persecute you, and shall say all manner of evil against you falsely, for my sake.”

Let’s move on to other news as I never intended for this blog to be a political sideshow.


New Car

No, we haven’t bought a new car.  But in a few years when our 11-year old Nissan Cefiro is ready for a replacement, I think I’ve found the perfect replacement.  It is cheaper than a Mini and a Mercedes Smart car, cuter than a Yaris, and fits in the narrow sois of Khrungthep without any problem.


I’m thinking white is the better choice of colors, but red is pretty cute, too.

We have had a lot of guests in town this weekend.  Thankfully, we had time on Thursday evening to have dinner with PJ and Theresa.  This was the first time we had seen PJ in years and years and our first time to meet Theresa, which was wonderful.  Unfortunately, no pictures.

I think Biing is in town with his family, although have not heard from him yet.  Otto and Han are in town with their friends.  Coincidentally, we ran into them at the front of the Oriental Hotel last night when we jumped out of a taxi to attend a wedding.  An entry about that beautiful event soon.

Also, Paul (aka “Ekin” here on Xanga) is in town although we don’t expect to hear from him necessarily.  Who else?  Oh, the brother of Trish’s close friend and colleague is in town from Hong Kong, staying in his vacation home.  Maybe we’ll see him Tuesday night if time allows.

Crazy, huh?

I’ll write about the wedding tomorrow but want to share with you some pictures from the demolition of the block of shop houses at the corner of Ploenchit Road (different stretch of Sukhumvit) and Witthayu/Wireless Road.  They are taking a long time to demolish these buildings, dismantling them from the inside.


In the picture above, you can see a low building with a metal roof in the midst of the empty lot.  That’s the “housing” for the workers, all of whom are from the countryside and many of whom are probably from Laos, Cambodia or Burma.  Seems to be the same story everywhere: immigrants from somewhere else come in to do the dirtiest, lowest-paying work.  Who does that work in their own countries?


Above, you can see how the building is being taken down, story by story.  An arduous process to say the least!


Interestingly, the buildings are brought down in separate sections.  The buildings to the left and right of this one are still being demolished, whereas this one is already completely gone.  I wonder why they do it this way?