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the realities

environment and energy
the trade off
DME campaign
growth and congestion
the calf path
road reserves - where are they
silent cops or silent executioneers

what to do

continuous flow methods
intersection design
lev el of service
context sensitive design
ways to reduce congestion toolbox
2007 comments re NMBM dedicated bus routes
2011 quo vadis BRT
letters and press articles

suggestions

Suggested Plan for Buffelsfontein/17th Ave zz
Sugges ted Plan for Walmer Heugh and Main Roads
S uggested Plan for Walmer Blvd / M4
Sugges ted Plan for William Moffat Expressway
Suggested Plan for Fairview By Pass
Sugge sted Plan for Walmer Park / Main Road
Suggested Plan for Uitenhage Road / N2
Suggested Plan for Stanford Rd / N2
Suggested Plan for Disa Ave / N2
Suggested Plan for Circular Drive and Kragga Kama
Suggested Plan for Samantha Rd / N2

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By Pierre Joubert
Bibliography


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Traffic Planning


Intersection design elements


read with continuous flow methods

executive summary

To reduce congestion and speed up traffic flow various intersection designs exist . Most of these are unconventional, uncommon and controversial, but are slowly gaining ground elsewhere. These designs include the median U-turn (MUT), jug handle, quadrant, super street, and continuous flow intersections.

All of these designs, through various geometric layouts and traffic control measures, employ indirect left turns, to eliminate direct right turns across oncoming traffic (turn left to go right). This allows deactivation of traffic signals and continuous flow along arterials.

Some of these options are described below and examples of how they can be applied in Port Elizabeth are shown in this presentation. The US AASHTO and SANRAL's Geometric Manuals, and other sources, variously describe these intersections.

An important option not included in SANRAL is the median U-turn (MUT). This form of intersection is common in some parts of America and can be seen elsewhere. Examples in Detroit and London are shown below. This form can be applied in Port Elizabeth along Uitenhage Rd, Stanford Rd and William Moffat, to great advantage.

One-way streets coupled with indirect turns and polygonal roundabouts, can be applied in certain locations (Langenhoven / Kempston). Polygonal roundabouts function as roundabouts, but are not necessarily round, and they have advantages over conventional roundabout shapes

indirect turns


The freeway cloverleaf flyover is an indirect turn, although not usually referred to as such, but mentioned here to help understand the concept of the median U-turn described next. This is a case of go left to turn right, but is very expensive and not proposed for PE at this time, except regarding the disused flyover on William Moffat

Certain elements of traffic movement called "conflicts" occur at intersections and are mentioned here

  • merging
  • weaving (crossing)
  • diverging

These conflicts occur in many places including the ends of the loops on a cloverleaf as pictured here. They also occur in the median cross-over. These conflicts need to be "managed" and are essential for intersections to work

The median U-turn (also called the median cross-over and the Michigan Left) is an indirect turn. This is another case of go left to turn right and has great potential for the NMBM in the opinion of the author, as described further down the page

The median cross-over intersection works like a roundabout in that traffic streams remain in motion while flow paths cross in a weaving pattern. The median cross over can be described as a flattened roundabout with a very long circulatory path. I t has greater capacity than a roundabout because it has more weaving capacity.

The end result of the MUT can also be said to be that it has the flow pattern of a flyover, but does not require the cost of a flyover bridge.





The median U-turn is extensively used in Detroit and elsewhere in the state of Michigan. The satelite photo on the right is of a portion of the Interstate 696 freeway that runs east west just north of Detroit and serves as a major arterial. Click on the picture for an enlarged view (110kb), To see a longer section of the 696 click here




The US FHWA manual describes the median cross-over in detail and can be read here from an extraction by Joubert

Examples of the median cross-over in London can be seen in satelite photos of an area east of Hyde park, which serves as a major arterial. Click on the picture for an enlarged view (173kb), and also notice a polygonal roundabout lower in the picture

Click here for a map of this London area (471kb)




This example of a polygonal roundabout is at Marble Arch. Note how busses travel in a nested roundabout pattern (136kb)









A further example of a polygonal roundabout in London can be seen at Wellington Arch. At this point a number of important London streets meet, and the large 5-lane polygonal roundabout about 190 meters across, allows one way continuous flow. Click on the picture for an enlarged view (167kb)

A similar polygonal roundabout is suggested by the author for the Langenhoven / Kempston / Burt convergence





A suitable place for median cross-overs to be applied in Port Elizabeth is on the Uitenhage road from N2 to past Kwadwesi, because of the wide median. This intersection style can be applied to the congested N2 flyovers, as shown on the diagram, and signals can be eliminated at:

There are other places where median cross-overs can be applied; eg Stanford Road, William Moffat, Buffelsfontein , Walmer Park, Circular Drive and Kragga Kama. In an adaptive way, one way major streets can serve a similar purpose where city blocks are used as circles. The concept can also be considered in built up areas of G Mbeki

Here is an example of how indirect turns can create continuous flow at Walmer Park's congested exits (click on pic)

Additional Comments

Note for authors of SANRAL Geometric Design Guidelines

The conflicts described above became serious in the case of full cloverleafs, as traffic volumes everywhere grew, because of the short distance between the ends of the loops. The result is that full cloverleafs have gradually been replaced by partial cloverleafs (Parclos). Examples can be shown later on request . The full cloverleaf at Coega has extra long "collector / distributor" lanes, which isolates it from the main N2, but the sections between the ends of the loops are still quite short and reduces its theoretical ultimate capacity. That will not be a problem in this case as the overall size of the interchange is so huge for the area it serves. However, full cloverleafs should not be constructed anywhere in RSA where traffic volumes are likely to be high, because they choke themselves when volume rises. It is unclear if this criterion was considered for Coega, but the Coega cloverleaf should not be an example to the rest of the country for this reason



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