Talk:Diamond interchange

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Adding Images

I removed the following code posted by 24.251.194.121:

[[Stack interchange between Arizona 101 Loop and 202 Loop alongside the Salt River at Tempe. Photo taken 04/22/05..jpg|thumb|350px|right|An aerial photo of a stack Interchange at Loop 101 and Loop 202.]]

If you wish to post an image, please make sure to use the correct filename. You can obtain a filename by finding the file on Wikipedia or Wikimedia and then copy/pasting the text that reads Image:blahblahblah.

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--Thisisbossi 21:52, 21 June 2006 (UTC)[reply]

Diamond Interchange and Michigan left?

The article states:

"In another, hypothetical, variation, a combination between a Michigan Left at grade intersection and the diamond interchange, all left turns from the minor road onto the freeway on-ramps can be eliminated. This could be done by implementing u-turn lanes on the freeway( assuming it is divided with a wide enough median). The u-turn lanes would be at least 400 metres away from the point where the on-ramps meet the freeway. Traffic that would usually wish to turn left onto the freeway would instead turn right in what would initially appear to be the wrong direction, drive until the u-turn lane, and make a u-turn, which would send them the same direction as an earlier left turn would have. The benefits of this type of intersection would be increased safety, as cars would no longer have to turn left across traffic to get onto the freeway. This is all hypothetical, as it has not been implemented anywhere."

This cannot be implemented on the main part of the freeway--are you referring to a U-turn on the main part of the freeway? Would the U-turn be a left-handed exit from the mainline? This tight U-turn setup exiting the mainline freeway on the left might not be desirable given the 75 MPH (120 km/h) design speed of most Michigan freeways. Also, the weaving from the on-ramp to the U-turn, crossing all that traffic, while not a problem on a signalized surface street (where most Michigan lefts are found), is not a good thing on a freeway.

Or, do you propose a Michigan left turn for the diamond interchange using frontage roads? This has already been done, in many places along I-96 and I-696 in metro Detroit, and also along I-94 in southern Macomb County. See [1] or [2].

The above examples are used where there is restricted land for the freeway. More commonly, the left turns are eliminated from the surface street by means of a Parclo A4. (Yes, Michigan has a fair amount of Parclo A4 interchanges.) --141.213.178.11 18:22, 22 December 2006 (UTC)[reply]

I read it as stating that Michigan Lefts may be along the minor street; not the freeway. Yes, the Parclo A4 is most common as it provides a more direct path, better accommodating driver expectancy; but it also requires left-turns across opposing traffic -- which may be undesirable in cases where there are few gaps available or specific safety concerns exist (for example, the bridge may have a steep grade, limiting sight distance). This may necessitate control of the left-turns through the use of a rotary or a signal, which will impact the operational efficiency and safety of the mainline minor street.

The use of Michigan Lefts removes the left-turn issue by relocating it beyond the interchange such that vehicles perform a U-turn, merging into oncoming traffic rather than crossing it; then weaving across the mainline of the minor street; and then diverging onto the ramp for the expressway.

The two particular concerns with this revolve around the merge and weave. Merging from a stop condition into the higher speed lane of the oncoming traffic can be an issue, but acceleration lanes can be provided to reduce this concern. The weave problem is addressed by a longer distance between the U-turn point and the ramps. Of course, the longer the distance, the more aggravation that motorists will have with having to travel out of their way; and more unfamiliar drivers may get lost. Pros and cons with everything: the great art of traffic engineering.

--Thisisbossi 19:50, 22 December 2006 (UTC)[reply]

Why does the article even include a hypothetical, non-existant intersection design? Looks more like someone trying to get an idea noticed, than anything that belongs in an encyclopedia.--Scottr76 06:15, 22 August 2007 (UTC)[reply]

Now that I agree with... that first line really irks me. Does the rest of it make sense, as I described above; or does it still seem confusing to others? If it is per my description, they are actually relatively common throughout the world. If it's something different, then I have no idea what it's describing and it may indeed be someone's own idea. This portion may benefit from some cleanup. --Bossi (talk • gallery • contrib) 11:20, 22 August 2007 (UTC)[reply]

It isn't hypothetical. Thika road (Nairobi to Thika) and the A104 going NW from Nairobi, have this design. Entering and crossing traffic turns left (driving on the left), then takes a U-turn to go the other way, or to leave on the other side. There are also full roundabouts on Thika Road, so crossing traffic not at a roundabout goes left to the first U-turn or roundabout, then back to a left turn onto the desired road. Robert Ullmann 17:07, 24 October 2007 (UTC)[reply]

I don't believe those roads are what the original editor intended, since they are not freeways, in which at-grade intersections are eliminated, since these roads involve several roundabouts. However, it is an interesting combination of Michigan lefts and roundabouts I hadn't seen before.

As for the hypothetical, as I see it, it would seem to involve a large amount of lane-shifting at high speed, both to the left and the right. It most likely would reduce accidents on the local road, but would create a far more dangerous situation on the freeway, and ultimately result in more serious injuries and fatalities from high-speed collisions, instead of the slower 'fender-bender' accidents that are more likely on the surface streets.

Whatever the case, I'm removing this passage, if only because of the POV that it would be safer, with no studies or even real-life situations that would show that to be true. The fact that it is merely hypothetical, as stated, shows that it really has no reason to belong in an encyclopedia at all.--Scottr76 03:12, 25 October 2007 (UTC)[reply]

The A104 doesn't have roundabouts, and so is closer to what (was) described. But your observation about safety is spot-on, the forced lane changes are a problem; although not as much as you might think, since other drivers anticipate them. Robert Ullmann (talk) 14:01, 22 March 2008 (UTC)[reply]