Highway Design Standards

[Mergingtraffic]

I've noticed lately, with several off ramp reconstructions and debuts here in CT, that it seems the new design standard is to not have a deceleration lane parallel to the mainline.  Al la here:

http://www.google.com/maps?f=q&source=s_q&hl=en&q=Interstate+691,+Cheshire,+Connecticut+06410&aq=&sll=37.0625,-95.677068&sspn=27.561629,56.162109&ie=UTF8&geocode=FQsdegIdJ7Cn-w&split=0&hq=&hnear=Interstate+691,+Cheshire,+Connecticut+06410&ll=41.557263,-72.923102&spn=0.000793,0.001714&t=h&z=19

Instead the ramp just breaks away from the main travel lanes.

Why is this?  Here it works fine, but in other areas, this set up seems to make traffic slow up in the mainline rather than a deceleration lane because drivers just see the ramp breaking away and feel they have to slow up ahead of it in the mainline travel lane.

[Zmapper]

I know Colorado almost always has separate acceleration and deceleration lanes, Wyoming is hit-or-miss, and the rest of the states I have been to seem to mostly forget about them.

[Brandon]

It seems to vary by state, and even DOT district.  Around here, InDOT Calumet Region uses them, IDOT District 1 does not, nor does the ISTHA.

[roadfro]

Generally speaking, it is better to have a longer ramp than a parallel deceleration lane. Ideally, all traffic on the mainline should be moving at about the same speed and not trying to slow down along the mainline unless absolutely necessary.

From the photo, it appears that the distance between the theoretical gore (the painted tip of the ramp and edge line) and the physical gore (the tip of the pavement edges / where the landscape starts) is quite long...perhaps this somewhat makes up for the lack of deceleration lane CT drivers are used to?


Nevada rarely uses deceleration lanes. I can only think of one off hand (I-80 WB at the West Fernley exit), and that's due to a design constraint for the ramp.

[J N Winkler]

Generally speaking, it is better to have a longer ramp than a parallel deceleration lane. Ideally, all traffic on the mainline should be moving at about the same speed and not trying to slow down along the mainline unless absolutely necessary.

But isn't provision of a parallel deceleration lane a cheap way of retrofitting queuing capacity without shifting the ramp onto a completely new alignment?

[agentsteel53]

But isn't provision of a parallel deceleration lane a cheap way of retrofitting queuing capacity without shifting the ramp onto a completely new alignment?

what about jersey-barriering the queueing ramp?  there should be enough shoulder room to, say, start with 12 feet lane and 10 feet shoulder and retrofit to 2 feet barrier, 4 feet median, 12 feet exit lane, 4 feet shoulder.  it would require only a restriping (and perhaps a filling of the rumble strip) - much less costly than building an entire new ramp.

this leaves the mainline with no shoulder until the exit, but it will cut down on people attempting to get into the exit lane late (a jackass behavior if I ever saw one), or deciding they screwed up and swerving back onto the mainline (not quite as jackass but still dangerous to merge from a slow exit lane into full-speed traffic).

[vdeane]

At least it's a deceleration lane and not an acceleration lane!

http://maps.google.com/?ie=UTF8&hq=&hnear=Rochester,+Monroe,+New+York&ll=43.078731,-77.622474&spn=0.00565,0.009624&t=h&z=17

[agentsteel53]

now, if you want to see horrific acceleration-lane design, check out the onramp from Mira Mesa Blvd westbound to I-805 southbound.  I know this one well, as it is about 1/4 of a mile from my work (a 45 minute commute in the worst of times)

http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=scranton+rd+at+mira+mesa+blvd,+92121&aq=&sll=43.077618,-77.622555&sspn=0.001579,0.005429&ie=UTF8&hq=&hnear=Mira+Mesa+Blvd+%26+Scranton+Rd,+San+Diego,+California+92121&ll=32.890928,-117.206987&spn=0.000908,0.002714&t=k&z=19

note the three lane on-ramp immediately converging down to one lane.  the mainline is 3 lanes southbound (and one false lane - concrete makes it look like it is a lane, but you realize that it quickly closes off on you and you have to merge over one more.  You end up with four lanes going southbound.

So essentially you get a 4-lanes-to-1 merge in the span of maybe 1000 feet, with the first two dropping within 300 feet.  Add to that that the whole structure is going uphill - you basically have to merge three lanes into full-speed traffic immediately, and then floor the shit out of your accelerator, praying that your latest rental car can go from 0 to 60 before you're creamed from behind by an 18 wheeler.

Atrocious.

[hm insulators]

Jake, if you think that's bad, try one of those "scrunched cloverleafs" (my term for them) on I-10 through Alhambra, Monterey Park, Rosemead and El Monte on for size sometime! Those are scary, both getting on and off the freeway! And there's never been any interest AT ALL in fixing the damn things!

[agentsteel53]

Jake, if you think that's bad, try one of those "scrunched cloverleafs" (my term for them) on I-10 through Alhambra, Monterey Park, Rosemead and El Monte on for size sometime! Those are scary, both getting on and off the freeway! And there's never been any interest AT ALL in fixing the damn things!

those are indeed quite low speed - but at least a lot of them appear to have c/d roads to go with them, as opposed to the expectation that you're supposed to hit the mainline and accelerate to the relevant speed within 3-400 feet.

and there are two 2->1 merges, (first when the ramp hits the c/d lane, and then when the c/d lane hits the mainline), as opposed to a single 4->1, which I claim is a whole hell of a lot worse.

[hm insulators]

Jake, if you think that's bad, try one of those "scrunched cloverleafs" (my term for them) on I-10 through Alhambra, Monterey Park, Rosemead and El Monte on for size sometime! Those are scary, both getting on and off the freeway! And there's never been any interest AT ALL in fixing the damn things!

those are indeed quite low speed - but at least a lot of them appear to have c/d roads to go with them, as opposed to the expectation that you're supposed to hit the mainline and accelerate to the relevant speed within 3-400 feet.

and there are two 2->1 merges, (first when the ramp hits the c/d lane, and then when the c/d lane hits the mainline), as opposed to a single 4->1, which I claim is a whole hell of a lot worse.

There are no c/d roads on a lot of those (the interchanges were built in the early 1950s), unless new ones were built in the last couple of years--it's been a while since I've been that direction, since when I drive to the L.A. area, I use the 210 as I'm heading towards Pasadena and the Valley. That being said, you can rest assured I'm in no hurry to try the onramp you're talking about. 

[agentsteel53]

There are no c/d roads on a lot of those (the interchanges were built in the early 1950s), unless new ones were built in the last couple of years--it's been a while since I've been that direction, since when I drive to the L.A. area, I use the 210 as I'm heading towards Pasadena and the Valley. That being said, you can rest assured I'm in no hurry to try the onramp you're talking about.  

I must've pulled up on google maps a different segment of the 10 than what you refer to.  indeed, now I seem to recall a few of those "oblong clovers" leading directly onto the mainline.  pretty bad, yes.  not quite as bad as the Templin Highway on-ramp onto i-5 south, with a stop sign, 200 feet of acceleration, and an impact barrier - but that, mercifully, has been fixed.  (it was a temporary measure anyway - it just so happens that Caltrans's idea of "temporary" is "six years".)

[Mergingtraffic]

or on CT-8 SB in Waterbury, CT, you have a left and right on-ramp AT THE SAME TIME, each with NO acceleration lane.   and the one on the right comes from a major interstate!
Plus, as the google link, shows a left and right merge sign on both sides of the road.

http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=waterbury,+ct&aq=&sll=43.076651,-77.624052&sspn=0.001552,0.004275&ie=UTF8&hq=&hnear=Waterbury,+New+Haven,+Connecticut&ll=41.55088,-73.051835&spn=0,0.002138&t=h&z=19&layer=c&cbll=41.55092,-73.051964&panoid=Ji5_SkBbZ8ZIfElUZMRjHQ&cbp=12,109.17,,0,13.44

C'mon even in 1968, the planner couldn't see a problem with this?!
At least this whole area is slated to be converted into a 4-level stack by 2030.

[Alps]

MUTCD allows both taper-type and parallel-type entrances and exits. If you want to see taper-type entrances done right, look at the NJ Turnpike. The key is the amount of distance from the physical gore and/or theoretical gore until your merging lane is too narrow to support separate traffic (around 10 feet, +/- a foot depending on truck percentage). That distance should be similar for either type of lane, otherwise you're doing it wrong and need a yield/stop condition. Taper-type decels, IMO, work best when you have a high-speed ramp or a curve far enough from the mainline that you can maintain a safe speed until exiting. Not so great on a cloverleaf loop, for example. Taper-type accels are not something I favor in general, so I can't speak as to when they're preferable.

[JREwing78]

I-94 in Parma, MI, about 10 miles east west of Jackson:
http://maps.google.com/maps?q=parma,+mi&oe=utf-8&client=firefox-a&ie=UTF8&hq=&hnear=Parma,+Jackson,+Michigan&gl=us&ll=42.269179,-84.598675&spn=0.003481,0.003744&t=k&z=18

The very short on-ramps are uphill, and are only about 800ft long. There's no lane to provide more time to merge into traffic. Most cars struggle to hit 60 before reaching the end of the on-ramp, and the freeway is posted for 70 cars/60 trucks.

The next exit to the east used to be worse - the on-ramps are just as short, but headed EBD the consequences were worse - there is a bridge immediately east of the merge point, and if you can't merge, you risk wrecking your car on the bridge's guardrail.

http://maps.google.com/maps?q=parma,+mi&oe=utf-8&client=firefox-a&ie=UTF8&hq=&hnear=Parma,+Jackson,+Michigan&gl=us&ll=42.268594,-84.53464&spn=0.001741,0.002894&t=k&z=19

Fortunately, this was rectified in 2008 or 2009. Google hasn't updated its imagery to reflect this.

[CL]

This monstrosity kills me. Here we're looking at the eastbound lanes of I-80 in Salt Lake City, just east of the otherwise marvelous I-15/I-80/SR-201 interchange. That southernmost lane comes from northbound I-15. The rest of the four lanes are I-15, I-80 and SR-201 traffic, which a few hundred feet back just merged together. Well, that southernmost lane defaults onto State Street, 300 or so feet after it merges onto I-80! This section of road is a nightmare to navigate. Northbound I-15 traffic wants to get onto I-80, but you have a mountain of SR-201, I-15, and I-80 traffic that needs to exit on State Street. I don't think I made sense, but if you take a look at it the aerial image will speak for itself.

[myosh_tino]

note the three lane on-ramp immediately converging down to one lane.  the mainline is 3 lanes southbound (and one false lane - concrete makes it look like it is a lane, but you realize that it quickly closes off on you and you have to merge over one more.  You end up with four lanes going southbound.

So essentially you get a 4-lanes-to-1 merge in the span of maybe 1000 feet, with the first two dropping within 300 feet.  Add to that that the whole structure is going uphill - you basically have to merge three lanes into full-speed traffic immediately, and then floor the shit out of your accelerator, praying that your latest rental car can go from 0 to 60 before you're creamed from behind by an 18 wheeler.

Atrocious.

With the advent of metering lights, the 3-down-to-1 on ramps are becoming more prevalent.  Especially on older ramps that are widened to accommodate the ramp meters.  One prime example up here in northern California is the on-ramp from De Anza Blvd to southbound I-280.

http://www.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=cupertino,+ca&aq=&sll=37.0625,-95.677068&sspn=40.001301,62.578125&ie=UTF8&hq=&hnear=Cupertino,+Santa+Clara,+California&ll=37.334034,-122.031037&spn=0.003474,0.003819&t=k&z=18

The ramp was short to begin with and had 2 lanes.  When ramp meters were installed, a third lane (the HOV lane) was added but nothing else was done to the ramp.  If you pan to the right on the google maps link I provided above, you'll also notice the incredibly short merging distance (approx 150 ft) you have to merge and get up to freeway speeds (65-70 MPH).

[mightyace]

this leaves the mainline with no shoulder until the exit, but it will cut down on people attempting to get into the exit lane late (a jackass behavior if I ever saw one), or deciding they screwed up and swerving back onto the mainline (not quite as jackass but still dangerous to merge from a slow exit lane into full-speed traffic).

I've seen drivers in Nashville cross 4 lanes of traffic to exit late! 

[InterstateNG]

I-94 in Parma, MI, about 10 miles east of Jackson:

I-94 from Albion to Jackson is very substandard.

[hm insulators]

this leaves the mainline with no shoulder until the exit, but it will cut down on people attempting to get into the exit lane late (a jackass behavior if I ever saw one), or deciding they screwed up and swerving back onto the mainline (not quite as jackass but still dangerous to merge from a slow exit lane into full-speed traffic).

I've seen drivers in Nashville cross 4 lanes of traffic to exit late! 

That's a common Los Angeles trick.

[mightyace]

Ah, must be those LA Music Execs in town to visit their Nashville offices, then! 

[Some_Person]

At least it's a deceleration lane and not an acceleration lane!

http://maps.google.com/?ie=UTF8&hq=&hnear=Rochester,+Monroe,+New+York&ll=43.078731,-77.622474&spn=0.00565,0.009624&t=h&z=17

I second that; take a look at this freeway-to-freeway ramp and the length of its acceleration lane http://goo.gl/maps/fNPZh