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Old 07-11-2019, 06:20 PM
 
Location: Georgia
5,243 posts, read 4,287,218 times
Reputation: 3030

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Traffic around the I-85/Clairmont interchange is a complete cluster, especially in the evenings. The main backups are from 85 northbound and Clairmont southbound. 85 traffic spills back all the way to the interstate, and I've seen Clairmont traffic back up as far as Buford Highway!

I see some big problems with the design. One is that all this traffic is trying to squeeze into just two lanes going south on Clairmont. Another is that 85/Clairmont is a fairly standard intersection but with U-turns and access roads. It's a very common design you see in places with a lot of access roads such as in Texas. Third, the intersection just south at Briarcliff forms a bottleneck.

This whole area needs to be reworked.

1. The offramp from 85 northbound needs to be moved back at least half a mile. Too much merging and weaving in a short distance. Wouldn't hurt to move back the onramp to I-85S either.

2. Forbid left turns on or off Clairmont between 85 and Briarcliff. I don't care how much of a hassle that creates for businesses and residents there, those left turns are just adding to the problem. They can do U-turns elsewhere on Clairmont.

3. Right turns from 85N access to Clairmont southbound needs to be two lanes with full right-of-way with a "keep moving" arrangement. This means we've now got 4 southbound lanes on Clairmont, but because of #2, we can use the existing center turn lane as one of these southbound lanes. End the rightmost lane at some distance south of 85.

4. Turn Briarcliff into an overpass over Clairmont. It will have to be seen whether this bridge should be just north or just south of the current alignment.

5. Build a short connecting road between Clairmont and Briarcliff with traffic lights on both ends like this. This would either go southeast or southwest of the existing intersection.

Another option is to rework the 85/Clairmont interchange itself, but the access roads limit the options. DDIs are out of the question; SPUIs could work.

This would take a lot of resources and would displace some businesses, but this stretch of Clairmont (and Briarcliff) has got to get fixed.

Last edited by Beretta; Today at 07:37 AM.. Reason: see DM
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Old 07-11-2019, 06:33 PM
 
2,064 posts, read 828,171 times
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For the access roads, what I notice they do in Texas (because every single highway has them there) is the offramp exits the highway about 1/2 mile to 1 mile before the actual interchange, sometimes this is as far back as the previous interchange. Then right before the interchange, you get an on ramp to merge into the freeway so you can completely skip the stop light and then exit off to the access road for the next interchange right after the overpass. It's completely the opposite of what I've seen anywhere else and I was trying to figure out why they did that because you have to travel down the access road for quite a way sometimes before you can get on the highway but I am believing this is the reason.

Basically the setup allows you to merge onto the access road with plenty of time before the interchange. Then traffic that was already on the access road has the option of skipping the stop light by merging onto the freeway right before the interchange, and even exit back onto the access road after the interchange if their destination is on the access road. At first I hated it but now I see why it's like that.

Also DDI's can work at interchanges with access roads. What they do is build slip lanes for the access road which will continue for thru traffic - these lanes are grade separated from the intersection (typically temporarily aligning with the submerged or elevated freeway) before returning to the access road after the DDI. The rest of the DDI functions as a normal one would.

This is the one exception that I would say the DDI would probably be much better than a SPUI at those intersections. SPUI's with access roads (like seen on PIB) just have too many signal phases.

Last edited by Need4Camaro; 07-11-2019 at 06:47 PM..
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Old 07-11-2019, 07:52 PM
 
Location: ATL -> HOU -> DAL
4,395 posts, read 3,560,598 times
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Quote:
Originally Posted by Need4Camaro View Post
For the access roads, what I notice they do in Texas (because every single highway has them there) is the offramp exits the highway about 1/2 mile to 1 mile before the actual interchange, sometimes this is as far back as the previous interchange. Then right before the interchange, you get an on ramp to merge into the freeway so you can completely skip the stop light and then exit off to the access road for the next interchange right after the overpass. It's completely the opposite of what I've seen anywhere else and I was trying to figure out why they did that because you have to travel down the access road for quite a way sometimes before you can get on the highway but I am believing this is the reason.

Basically the setup allows you to merge onto the access road with plenty of time before the interchange. Then traffic that was already on the access road has the option of skipping the stop light by merging onto the freeway right before the interchange, and even exit back onto the access road after the interchange if their destination is on the access road. At first I hated it but now I see why it's like that.
It helps the intersections themselves a lot because like you said, traffic can just skip the light. It's not like everyone is forced through the light.

I've been using it a lot during my commute. Exit onto the frontage road, stay in the left lane, head back onto the interstate, usually just stay in an exit only lane, then exit again, repeat. It's definitely faster when the freeway is backed up.
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Old 07-11-2019, 09:43 PM
 
Location: Georgia
5,243 posts, read 4,287,218 times
Reputation: 3030
Quote:
Originally Posted by Need4Camaro View Post
For the access roads, what I notice they do in Texas (because every single highway has them there) is the offramp exits the highway about 1/2 mile to 1 mile before the actual interchange, sometimes this is as far back as the previous interchange. Then right before the interchange, you get an on ramp to merge into the freeway so you can completely skip the stop light and then exit off to the access road for the next interchange right after the overpass. It's completely the opposite of what I've seen anywhere else and I was trying to figure out why they did that because you have to travel down the access road for quite a way sometimes before you can get on the highway but I am believing this is the reason.

Basically the setup allows you to merge onto the access road with plenty of time before the interchange. Then traffic that was already on the access road has the option of skipping the stop light by merging onto the freeway right before the interchange, and even exit back onto the access road after the interchange if their destination is on the access road. At first I hated it but now I see why it's like that.

Also DDI's can work at interchanges with access roads. What they do is build slip lanes for the access road which will continue for thru traffic - these lanes are grade separated from the intersection (typically temporarily aligning with the submerged or elevated freeway) before returning to the access road after the DDI. The rest of the DDI functions as a normal one would.

This is the one exception that I would say the DDI would probably be much better than a SPUI at those intersections. SPUI's with access roads (like seen on PIB) just have too many signal phases.
Dallas does this. Almost to a fault. They basically switch where you'd expect to see entrances and exits.

But there is much less weaving, because you have a full mile or so on the access roads to get in the lane you want.

I'm not familiar with DDI's with access roads--can you give an example?
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Old 07-11-2019, 09:53 PM
 
2,064 posts, read 828,171 times
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Quote:
Originally Posted by toll_booth View Post
Dallas does this. Almost to a fault. They basically switch where you'd expect to see entrances and exits.

But there is much less weaving, because you have a full mile or so on the access roads to get in the lane you want.

I'm not familiar with DDI's with access roads--can you give an example?
N Interstate 35 Frontage Rd
https://maps.app.goo.gl/zMNWa8cWmGt61mjZ8

Posting from my phone so I cant give a whole ariel view. This one isn't too far from where I live. Basically the access roads have a slip lane that travel under the overpass of the DDI for traffic that intends to continue straight while the traffic that continues toward the DDI must turn.

So basically you 'exit' the access road, share the same ROW with the interstate, grade separated from the entire interchange and re-enter the access road on the other side of the interchange.

I personally do not like how Texas uses access roads on every single highway but I will have to say this instance was pretty creative.

Last edited by Need4Camaro; 07-11-2019 at 10:03 PM..
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Old 07-11-2019, 10:04 PM
 
Location: Georgia
5,243 posts, read 4,287,218 times
Reputation: 3030
Quote:
Originally Posted by Need4Camaro View Post
N Interstate 35 Frontage Rd
https://maps.app.goo.gl/zMNWa8cWmGt61mjZ8

Posting from my phone so I cant give a whole ariel view. This one isn't too far from where I live. Basically the access roads have a slip lane that travel under the overpass of the DDI for traffic that intends to continue straight while the traffic that continues toward the DDI must turn.

So basically you 'exit' the access road, share the same ROW with the interstate, grade separated from the entire interchange and re-enter the access road on the other side of the interchange.
Okay. Not bad but this would take up way too much space in the 85/Clairmont area.

Quote:
I personally do not like how Texas uses access roads on every single highway but I will have to say this instance was pretty creative.
Agreed.
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Old 07-11-2019, 10:12 PM
 
2,064 posts, read 828,171 times
Reputation: 1658
Quote:
Originally Posted by toll_booth View Post
Okay. Not bad but this would take up way too much space in the 85/Clairmont area.



Agreed.
I guess SPUI would be the only other way if it cant be condensed a bit. I like the SPUI's on PIB but I think a DDI will outclass them if they could be implemented.
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Old Yesterday, 10:00 AM
 
Location: Georgia
5,243 posts, read 4,287,218 times
Reputation: 3030
Quote:
Originally Posted by Need4Camaro View Post
I guess SPUI would be the only other way if it cant be condensed a bit. I like the SPUI's on PIB but I think a DDI will outclass them if they could be implemented.
Another problem with DDIs is that they are not well-suited to large volumes of through traffic on the weaving road. Only one direction can get all green lights at a time.

A SPUI similar to the ones on PIB could work. Left-turning cars take longer to clear the intersection however.
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Old Yesterday, 10:19 AM
 
Location: Kirkwood
23,393 posts, read 17,563,923 times
Reputation: 5423
I agree that something needs to be done about this interchange.
Both 1/2 penny
http://www.dekalbtransitmasterplan.c...rio_Poster.jpg
and full penny
http://www.dekalbtransitmasterplan.c...rio_Poster.jpg
propose running ART on Clairmont Rd.

Last edited by Beretta; Today at 07:37 AM.. Reason: copyright
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Old Yesterday, 11:45 AM
bu2
 
10,002 posts, read 6,438,148 times
Reputation: 4156
Quote:
Originally Posted by sedimenjerry View Post
It helps the intersections themselves a lot because like you said, traffic can just skip the light. It's not like everyone is forced through the light.

I've been using it a lot during my commute. Exit onto the frontage road, stay in the left lane, head back onto the interstate, usually just stay in an exit only lane, then exit again, repeat. It's definitely faster when the freeway is backed up.
Texas has been shifting to this model in busy areas. The older access roads have the entrance after the intersection.
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