tsubba - 28 January, 2008 | Bangalore

A quick study and some rough sketches to understand the cauvery junction underpass. Looks like the palike will need more than the magic of precast boxes to make this work.

Existing situation
The image to the right shows the the existing situation at the junction. Vehicles going south to north and north to south travel straight along the bellary road. Vehicles from south going to the west take left on the slip road. Vehicles from west going north from west take left on the slip road.
The main conflict is between north-south traffic and vehicles from the west going south towards the city. When vehicles from west heading south towards the city crosses the road, both north bound and south bound traffic has to stop.
The palace maidan is to the right of bellary road. 1st main and 18th cross are two other roads in the area. Both sides of sankey road and the western side of bellary road are lined with business.

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Ideal solution
If some space to the east of bellary road, on palace maidan was available, then true signal-free fast movement through this junction would have been possible. One such ideal solution is shown in the figure to the right. Simply  lift the problematic linkage and ease it into the south bound traffic. The main linkage now has to only deal with merging and exiting traffic.
Other variations to the theme, like underpass or a loop that bends backwards etc are also possible. But as idontspam has pointed out acquiring palace lands is not possible.

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An idealized alternative
Thus, it is not possible to separate the problematic west-south linkage by physically moving it. One way out is to move the main link iself and that is what, I think, the palike is trying to do. An idealized version of this solution is shown to the right. It is not so much a U-turn as it is going through a circle or roundabout - only, in this half circle, there is traffic at the center of the circle instead of somebody's statue.

South bound traffic still has stop to let traffic from west going south, to cross the bellary road. S1 and S2 will be two signals regulating usage of the road. When S1 is green S2 is red and vice-versa.




The bottomline of this design is that the north bound traffic(towards airport) gets continuos free movement by passing over all cross-traffic.

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Actual implementation
But the success of this alternative rests entirely on the geometry of the actual implementation.




From what I have read on this underpass so far, all traffic headed to hebbal  has to take this loop. Providing a nice little  loop for single lane is fine, but for the entire throughfare on this road to take the loop, the loop better be wide with a nice and smooth curvature. If the turn radius is too small or the bend too sharp, then this design will seriously slow down north bound traffic. Hopefully the palike has paid enough attention to it.
     
Further, since actual turning on the loop happens on the underpass the question is, is the underpass long enough to ensure smooth u-turns? Also, is there enough space on sankey road to allow smooth u-turns? As is the norm for palike, no details were avaialable on the its website. And as is the norm for papers, no useful information was available in their reports. But a report I read in the Hindu, fleetingly mentioned that the vehicles are to take the u-turn after travelling 50 meters on Sankey road. Based on that, I drew a rough approximation of a possible design. The red rectangles are of approximately the same size as the buses seen near the top right of the image. I dunno if the acual palike design is similar to this, but this figure highlights the challenges at Cauvery Junction.

The issue here is that deeper the loop goes into sankey road, away from bellary road, more difficult the turn becomes. On the other hand, if the loop is too close to the bellary road, then there will be no place for underpass traffic to emerge. As you can see the loop is real tight and traffic will have to slow down. That is OK as long as it keeps moving.

If the loop gets too tight, one alternative is to use this loop as a slip lane for north bound vehicles. When S2(see pic above) is green, they could make S1 and S3 red. Vehicles at S3 then have the option of taking the loop to avoid waiting for S2 to go red. When S2 is red, they just continue straight. That way, irrespective of the signal, vehicles at S3 always have an option to keep moving. Though all this switching could get messy and very confusing.
The palike will have to walk on a very tight rope to balance all this. It will be very difficult to make all this work. And I have not even said anything about the width of the underpass to carry all traffic from west headed to either north or south.

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