Archive for March, 2011|Monthly archive page

The Diverging Diamond Interchange

Ok – Here’s my take on the DDI.  I’ve included the email correspondence (with permission) from Gilbert Chlewicki, P.E., who is the President of Advanced Transportation Solutions.

 

Hi Gilbert,

Very interesting website.  I first became aware of the DDI thanks to a post on InjuryBoard.com, where I’m a contributing author.

The particular post is at: http://stcloud.injuryboard.com/automobile-accidents/st-clouds-diverging-diamond-traffic-safety-or-175-mllion-boondoggle.aspx?googleid=287742

I’ve been by the one on I-44 in Missouri many times, but haven’t been through it yet.

I watched the videos with the post, and gave your website a thorough read.

Here are my concerns:

1.  There’s no provision for proceeding back onto the original highway after exiting.

Truck drivers often pull off on ramps to check maps, answer calls etc. Also, they sometimes take the wrong exit.  With this design, there’s no way to get directly back onto the highway again.  This is something that can have very serious consequences in a truck.

Also, sometimes overdimensional or overweight permitted loads sometimes need to use these ramps to get around a low clearance, or weight limited bridge.

2.  The angle of the roadways shown is insufficient to give a semi truck adequate visibility to the right to safely make the left turn. The second attached file (left turn1.jpg)  shows where I’m talking about.
.

How would you address these issues?

Your comments to the post at InjuryBoard would also be welcome.

Thanks,

Truckie-D

Thanks for writing Truckie-D.  Let me try to address your issues as best as possible.

1) No thru movement for ramp traffic – This is true and it is the one small set back of the design.  But for all the benefits that the design has, it is usually worth that one drawback, since the only people who generally need to go straight are people who get off a wrong exit.  But it is only a small inconvenience for most drivers to simply go to the next intersection and make a u-turn.  Admittedly, this movement is a little more inconvenient for truckers.  At the I-44/SR 13 DDI, I saw firsthand that it wasn’t that hard for trucks to make a u-turn at one of the nearby intersections or at a gas station.  There is a large volume of truck traffic through that interchange on both I-44 and SR 13 and that was taken into consideration.

A DDI shouldn’t be designed where there are clearance issues on the highway for trucks.  Any half decent engineer should realize this when designing the interchange.  So I wouldn’t worry about clearance issues at a DDI to force ramp thru movements.

With all that being said, I have designed a DDI that does allow thru movements for the ramps.  It is called an Expanded Diverging Diamond Interchange.  This would allow the use of a DDI in a place like Texas where the service roads next to the interstate will require some sort of thru movement.  The only issue with the Expanded DDI is that in order for it to be an efficient design, it needs a lot more land; at least the size of a full cloverleaf interchange.  And it is still a bit limited on how much ramp traffic that wants to go straight will be able to get through the intersection before the design becomes inefficient.

2) Visibility for left turns – The sketch that I have on the website is just to show clearly where each traffic movement goes.  When designed in real life, the angle takes into account sight distance issues.  Next time you drive through Springfield, MO take the opportunity to go through the DDI and make a left.  You’ll see that it is very easy and that everything has been designed for large trucks to make the turns easily and see the traffic very clearly.  For other DDIs, where the left turns are a little more difficult, a traffic signal will sometimes be placed for these lefts.

Please feel free to post these comments on to your blog.  If I can assist you in any other way, please let me know.

Gil

Hi Gil,

Thanks for your reply.

On Sun, Jan 23, 2011 at 7:37 PM, Gilbert Chlewicki <gchlewicki@atsconsults.com> wrote:

Thanks …

… But it is only a small inconvenience for most drivers to simply go to the next intersection and make a u-turn.  Admittedly, this movement is a little more inconvenient for truckers.  At the I-44/SR 13 DDI, I saw firsthand that it wasn’t that hard for trucks to make a u-turn at one of the nearby intersections or at a gas station…

I don’t consider this a minor item.  The company that I lease to has an extremely strict no u-turn policy – and with good reason. Doing one results in instant unemployment.  A u-turn in a semi is extremely dangerous. It exposes the side of the trailer to traffic, and has no underrun protection.  Any smaller vehicle hitting one tends to decapitate the occupants, which is generally frowned upon.  It also results in huge settlement costs.

Also prohibited, again with good reason, are turnarounds on private property.  Doing one is automatically considered as a chargeable preventable collision — even if there’s no damage.  Most of the time, things such as driveways aren’t designed for trucks, and a truck can cause severe pavement damage.  This can get very expensive very fast.   There may also be clearance issues with signs etc. that can be difficult to see.

A DDI shouldn’t be designed where there are clearance issues on the highway for trucks.  Any half decent engineer should realize this when designing the interchange.  So I wouldn’t worry about clearance issues at a DDI to force ramp thru movements.

Mr. Murphy constantly rears his head in the trucking industry.  If a bridge has n feet of clearance, someone will want a load moved through there that requires n+1 feet.

With all that being said, I have designed a DDI that does allow thru movements for the ramps.  It is called an Expanded Diverging Diamond Interchange.  This would allow the use of a DDI in a place like Texas where the service roads next to the interstate will require some sort of thru movement.  The only issue with the Expanded DDI is that in order for it to be an efficient design, it needs a lot more land; at least the size of a full cloverleaf interchange.  And it is still a bit limited on how much ramp traffic that wants to go straight will be able to get through the intersection before the design becomes inefficient.

I’d love to see a diagram of this. Can you send me one?

2) Visibility for left turns – The sketch that I have on the website is just to show clearly where each traffic movement goes.  When designed in real life, the angle takes into account sight distance issues.

Ok, that makes sense.

Next time you drive through Springfield, MO take the opportunity to go through the DDI and make a left.  You’ll see that it is very easy and that everything has been designed for large trucks to make the turns easily and see the traffic very clearly.  For other DDIs, where the left turns are a little more difficult, a traffic signal will sometimes be placed for these lefts.

I’ll try to get through there sometime and take a good look.

Please feel free to post these comments on to your blog.  If I can assist you in any other way, please let me know.

Ok, I’ll do that.  I will make one suggestion – that any of these interchanges that don’t allow thru traffic be signposted on the highway in advance of the ramp.  I really hate surprises like that.

Also, if you ever want a truck driver’s point of view on something, please feel free to email me.  I’m a safety fanatic, so I’m willing to do whatever I can to help.  If you’d like to know more about me, visit my blog, and take a look.

Thanks,

td

 

So, that’s the scoop.  I haven’t heard anything more from Gilbert about the through traffic version.

While it may be a good idea for use in limited circumstances, I’m afraid that I’m going to have to come down quite firmly opposed to the usage of the DDI design, because of the issues noted above.

 

td

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