Partial cloverleaf design

[tradephoric]

I-75 & Baldwin Road interchange in Michigan uses a partial cloverleaf design.  Traveling NB I-75 the cloverleaf is used as an exit ramp (NW cloverleaf) and traveling SB I-75 the cloverleaf is used as an entrance ramp (SW cloverleaf). 

http://maps.google.com/?ll=42.705177,-83.306419&spn=0.005282,0.008272&t=h&z=17&vpsrc=6

The question is what design is better?   I drive this stretch and I personally find the NB I-75 ramp setup to be much more convenient.  You don't have to wait to turn left at a red light like you do with the SB I-75 ramp setup.

[pianocello]

You don't have to wait to turn left at a red light like you do with the SB I-75 ramp setup.

True, but consequently, you have to wait even longer when going left onto I-75 (3-cycle light vs. 2-cycle).

[tradephoric]

The signal that services NB Baldwin traffic turning left onto the freeway is actually a 2-phase signal.  There are 2 separate signals that service the NB on/off ramps, setting up a superstreet design which essentially splits up Baldwin road into two one-way streets.

A huge problem for traffic engineers is trying to coordinate closely spaced traffic signals which is a natural occurrence at freeway to arterial interchanges.  Had a cloverleaf been used as the SB I-75 exit ramp instead of an entrance ramp (cloverleaf constructed on the SE corner instead of the SW corner), you'd be able to achieve perfect dual coordination along Baldwin Road at all times of the day. 

[Brandon]

Setup for partial cloverleafs?  I prefer the loop-off the freeway as opposed to the loop-onto the freeway.  Loop ramps tend to slow traffic down too much for entering, IMHO.  In addition, with a loop off the freeway, only one light per direction on the cross street is needed.  One for thru and opposing left onto the freeway in each direction.  Why should traffic stop twice?

[tradephoric]

Also i was pondering which setup is safer.

Loop-off the freeway:  2 off-ramps, and 1 on-ramp.
Loop-onto the freeway:  2 on-ramps, and 1 off-ramp.

My guess would be the more dangerous design is the Loop-onto the freeway where you have 2 on-ramps and 1 off-ramp.

[Brandon]

Also i was pondering which setup is safer.

Loop-off the freeway:  2 off-ramps, and 1 on-ramp.
Loop-onto the freeway:  2 on-ramps, and 1 off-ramp.

My guess would be the more dangerous design is the Loop-onto the freeway where you have 2 on-ramps and 1 off-ramp.

Agreed about safety.  One merge point has got to be safer than two, and one spot where traffic can cross the other stream on the crossroad instead of two has also got to be be safer.  Why are DOTs (IDOT, I'm looking at you) building two on ramps instead of two off ramps?

[Hot Rod Hootenanny]

Also i was pondering which setup is safer.

Loop-off the freeway:  2 off-ramps, and 1 on-ramp.
Loop-onto the freeway:  2 on-ramps, and 1 off-ramp.

My guess would be the more dangerous design is the Loop-onto the freeway where you have 2 on-ramps and 1 off-ramp.

Agreed about safety.  One merge point has got to be safer than two, and one spot where traffic can cross the other stream on the crossroad instead of two has also got to be be safer.  Why are DOTs (IDOT, I'm looking at you) building two on ramps instead of two off ramps?

They aren't the only ones. ODOT seems to be thinking the same way.
http://www.dot.state.oh.us/projects/I-270/Documents/NCOPhasing.pdf
(focus on the red line for US 23 at I-270)

[roadfro]

Also i was pondering which setup is safer.

Loop-off the freeway:  2 off-ramps, and 1 on-ramp.
Loop-onto the freeway:  2 on-ramps, and 1 off-ramp.

My guess would be the more dangerous design is the Loop-onto the freeway where you have 2 on-ramps and 1 off-ramp.

Agreed about safety.  One merge point has got to be safer than two, and one spot where traffic can cross the other stream on the crossroad instead of two has also got to be be safer.  Why are DOTs (IDOT, I'm looking at you) building two on ramps instead of two off ramps?

One merge point may be safer than two...at least from the freeway perspective. However, depending on the geometry of the cross street, there could be fewer conflict points in the other setup.

The loop-onto-freeway scenario (2 on-ramps) removes the left-turn onto the freeway from the arterial cross street, as each direction makes its entrance from the right lane. Assuming it's a signalized interchange, this has the benefit of dispersing the traffic that would otherwise enter the freeway as a platoon being released by the left turn signal (assuming protected lefts), which generally helps merging. This setup also has the benefit of removing the left turn phase on the cross street, which allows a lower signal cycle length which and can aid in signal progression on the arterial, if present.

[froggie]

Assuming it's a signalized interchange, this has the benefit of dispersing the traffic that would otherwise enter the freeway as a platoon being released by the left turn signal (assuming protected lefts), which generally helps merging.

In my experience, it doesn't really disperse traffic taking the on-loop unless traffic volumes are light to begin with.  In no small part because that 1st signal bunches the traffic up that's eventually taking the on-loop.

This setup also has the benefit of removing the left turn phase on the cross street, which allows a lower signal cycle length which and can aid in signal progression on the arterial, if present.

I don't see that as a benefit.  In the loop-off-freeway scenario, unless there are additional side streets involved (like frontage roads), you're only going to have one signal in each direction on the cross-street...and that's a simple 2-phase signal.  Unlike the loop-onto-freeway scenario where you'll have two signals in each direction on the cross-street...the only way this wouldn't be the case is if there aren't any additional side streets involved and you configure the left turn onto the side street as a left-side merge, like what was done recently at US 52/US 63 in Rochester, MN.

[tradephoric]

Here is a great example of an off-ramp loop setup in Woodhaven, Michigan:
http://maps.google.com/?ll=42.138364,-83.241445&spn=0.00498,0.008272&t=h&z=17&vpsrc=6

There is one signal that services the NB I-75 off ramp and stops only the EB direction of travel on the arterial road.  Also notice that with this design there is only one off ramp and one on ramp as the ramps are divided from the freeway itself.

[vtk]

I read in an ODOT study somewhere that loop offramps are discouraged in new designs because of the rapid deceleration required.  I can imagine some people arguing that this isn't a problem if the driver is paying attention, but just underestimating the ramp curvature a little bit can lead to a tough situation that's difficult to correct.  Trying to brake while already going around a curve too fast could push the tires past the limits of static friction, and then you're in a much worse place...

[Truvelo]

Correct me if I'm wrong but I believe it's been policy for a while in the US to prefer loop onramps. I can't remember where I read it but it was something to do with the possibility of stationary traffic forming on the off loops if there's there's congestion on the surface street and this could spill back onto the freeway itself with dangerous consequences. Therefore with two on loops this wouldn't happen except if there's an accident or other incident causing congestion on the freeway.

[Brandon]

Correct me if I'm wrong but I believe it's been policy for a while in the US to prefer loop onramps. I can't remember where I read it but it was something to do with the possibility of stationary traffic forming on the off loops if there's there's congestion on the surface street and this could spill back onto the freeway itself with dangerous consequences. Therefore with two on loops this wouldn't happen except if there's an accident or other incident causing congestion on the freeway.

Pardon me while I laugh my ass off.  Stationary traffic backs up onto the freeway from single, diamond-style off ramps all the time.  What's the difference between a loop ramp setup like this backing up and a single off ramp backing up?  Not a damn thing.

[tradephoric]

The addition of the extra off-ramp is adding more queuing space for vehicles exiting the freeway making it less likely backups will form onto the freeway, IMO.

There are a few potential problems that i can see with the two off-ramp design:

1.  If the diamond ramp is backed up onto the freeway, you will have some drivers that don't want to wait and cut back onto the freeway to take the loop ramp instead.  I have seen this happen first hand during the PM rush at the Baldwin interchange.  In the course of 1,400 feet, Baldwin Road goes from a 6-lane boulevard to a 2-lane road just north of the interchange.   This creates a major bottleneck and vehicles exiting off the diamond ramp will backup onto the freeway.  A percentage of impatient drivers  have found you can save a few minutes if you cut back onto the freeway to take the cloverleaf ramp instead and then make a u-turn on the arterial. 

2.  Multiple off ramps can cause confusion for motorists.  Any confused driver who doesn't know their north from their south may try to weave back onto the freeway (or weave into the offramp) at the last moment. 

3.  Vehicles need to go from freeway speeds to 25-35 mph quite rapidly to safely navigate the cloverleaf off-ramp.

All 3 potential problems are resolved with the Woodhaven design where the off-ramps are separated from the main freeway.

[vdeane]

Take this lovely interchange right here: http://maps.google.com/?ll=43.078778,-77.485446&spn=0.004906,0.009012&t=h&z=17&vpsrc=6

It's not uncommon for traffic in the right lane (each direction) to be using their brakes because a truck decided to get off here.  Those curves require traffic to move at about 30 mph on the ramps, and the speed limit on I-490 here is 65.

[Truvelo]

Take this lovely interchange right here: http://maps.google.com/?ll=43.078778,-77.485446&spn=0.004906,0.009012&t=h&z=17&vpsrc=6

It's not uncommon for traffic in the right lane (each direction) to be using their brakes because a truck decided to get off here.  Those curves require traffic to move at about 30 mph on the ramps, and the speed limit on I-490 here is 65.

Why couldn't a separate off ramp be added for I-490 West? There appears to be enough room to make it a five ramp junction.

[WNYroadgeek]

Looks like it used to be a full cloverleaf- you can see a ghost ramp in the bottom-right quadrant.