The following is a post by Karthik Shashidhar
In order to encourage use of public transport, it is important to provide good and safe footpaths – Karthik Shashidhar
Recently, Karnataka Chief Minister Siddaramaiah, following an inspection of projects around the city, made an announcement that the footpaths that are being built under the TenderSURE project are “too wide”, and that henceforth they will need to be narrower, allowing for greater road space. In making this statement, the chief minister betrays a lack of understanding of how traffic flows work, and the concept of thinking at the margin.
A journey between two points in a city can be modelled as an alternating sequence of intersections (“nodes” in graph theory parlance) and road segments (“edges” as per graph theory). It is intuitive to see that the total time taken for the journey is the total of the time taken to traverse each node and edge along the way. Thus, a city’s traffic can be modelled as a collection of such traversals across nodes and edges.
Image courtesy: Greg Younger
Elementary network theory tells us that the capacity of a network between two given points is defined by the capacity of the capacity of the “bottleneck” – that part of the network that has the least capacity. For those not well-versed with network theory, this is akin to the proverb that says that the strength of the chain is equal to the strength of the weakest link. Given the alternating flow though road segments and intersections, note that the bottleneck can occur at either a road or at an intersection.
An element of network design is that as long as bottlenecks remain, expanding capacity at non-bottleneck elements of a network is not going to increase capacity of the network – traffic will simply flow faster through the network element with expanded capacity, and then get stuck at the bottleneck.
Expanding capacity at bottlenecks is not a simple solution, either, for now the bottleneck can shift to a nearby network element whose lower capacity now becomes a new bottleneck! For example, after the Malleswaram circle underpass was built, new traffic hold-ups have been created at the Sampige Road – 8th Cross intersection, at the Margosa Road – MKK Road intersection and the MKK Road – Link road junction. When the Marathahalli Rail over-bridge was expanded (a necessary step, no doubt), traffic started piling up at the Marathahalli-Outer Ring Road junction, forcing a change in the topography of that junction (blocking off some right turns, etc.).
The extra-wide footpaths built under the TenderSURE project have no doubt reduced the capacity of the roads that they are adjacent to, which is what has prompted the adverse reaction from motorists, auto rickshaw drivers and now the Chief Minister. And looked at in isolation, they do seem like they have reduced the capacity of the network. However, if we take a holistic view, and look at these roads as simply edges in the traffic network of Bangalore, a reduction in capacity is not apparent.
A feature of Bangalore traffic, given the nature of the road network, is that bottlenecks are usually at the intersections, and not at the roads. As a consequence, irrespective of how much we widen the roads, the intersections will continue to constrain the flow of traffic in the city. In other words, making roads narrower will not have a material impact on the throughput of traffic in the city.
This might lead the reader to jump to the conclusion that if intersections are bottlenecks, their capacity should be eased, and thus grade separators (flyovers / underpasses) are the solution. Again, that demonstrates localised thinking, for, as described in examples above, while flyovers and underpasses might ease bottlenecks at those specific intersections, they simply end up shifting bottlenecks rather than eliminating them.
Given the network of roads in the city, and the consequent traffic flows, the best way to increase throughput of traffic is to improve utilisation of road space rather than getting rid of (currently existing) bottlenecks by means of road widening and grade separators. The obvious way to improve utilisation of road space is by getting more and more people to use public transportation, and increasing the capacity of the public transport network.
A feature (or perhaps “bug”) of public transport is that it doesn’t provide last mile connectivity, necessitating users to walk to/from the nearest bus or metro stops. Thus, in order to encourage use of public transport, it is important to provide good and safe footpaths, which is an objective of the current TenderSURE project. Existing footpaths in Bangalore are largely ineffective, given barriers such as trees, lampposts, transformers and parked vehicles. In this context, the footpaths that are being designed and built by the TenderSURE project are a landmark effort, and need to be encouraged.
Finally, where does thinking at the margin come in? The decision on how wide a footpath should be rests on a tradeoff between footpath space and road space, and this can be framed as follows – “given current road and footpath widths, is it beneficial to increase road width by a foot at the cost of footpath width?”
Globally, road width is governed by the width of a “car lane”, the width of road required to accommodate one passenger car. Efficient road usage comes out of making the width an integer multiple of this width of car lane – increasing width beyond that doesn’t increase road capacity.
Roads that are currently being developed under the TenderSURE project have been designed to allow for an integer number of “lanes” of traffic. Marginally increasing the width by a foot or two will not only have no impact on the number of lanes, but will also lead to inefficient usage of (increased) road space by providing road users room for breaking lane discipline. On the other hand, increase in footpath width directly translates to more available lanes for pedestrians, and thus directly increases capacity of the footpath.
Thus, the Chief Minister’s comments on footpath width are misguided and inappropriate. It is hoped that standards imposed by the current TenderSURE design prevail, and we will soon have a good network of footpaths in Bangalore city.
The Kannada translation of this post can be found here