The principles of Traffic Demand Management
This is a very complex issue and people have written books on the topic. Here I am going to simply attempt to provide a summary for the casual reader.
Essentially the equation is demand / capacity. The solution thus is to either reduce demand or increase capacity.
Typically, demand is reduced by way of congestion charging, road pricing, tax on vehicles, removal of subsidy (on petrol), limiting parking and removing paid parking. Sometimes the equation is tilted by prioritising buses, high occupancy vehicles and NMT.
Typically, capacity is enhanced by more road building. And we have dozens of case examples making this evidently a useless option. The term used to explain the failures is 'induced congestion', where after flyovers, under-passes or road widening, more vehicles start plying on them negating any proposed benefit.
Now there can atypical measures of changing the equation. Within the UK health service we had a crisis a short while ago over waiting times/lists. Initially this was attributed to lack of manpower. Root cause analysis showed that it was to do with queue management. Now take our railway ticket booking counters - we have 10 windows and 10 queues. But try remembering going to a Bank in the Western world; they have 10 counters but one queue!! Which works better? Research shows the latter works better.
This applies to our roads when we decide to use road space optimally. Recently I circulated a link to an article describing giving an experimental explanation of how traffic jams happen even without bottle necks. This is simply because every road has a given capacity for a specified speed limit. Motorways have a sped limit of 70 miles per hour, but at that speed, stopping distances / 2 second gap is 315 feet. When more cars exceed capacity but attempt the above speed, chaos begins, usually there is a rear end collision and this takes out a lane causing further delay. To improve reliability and also get more people to their destinations safely and quickly, motorways’ now have two implements - 1) Ramp / slip road meters (signals) - these will restrict number of vehicles on motorway at a given point in time. 2) Variable speed limit - thus when it starts getting saturated, the speed limit is dropped to 50 (or even 40), and this reduces the 2 second gap to 175 or 120 feet respectively, allowing more cars to travel reliably.
Now a motorway connects 2 cities with no habitat in between, hence traffic can be handled more easily.
The city is rather different. It’s only natural that after every 100 yards there will be a cross road or a T junction. Our planners (and urban residents) need accepting this fact. Because right now all they seem to want to do is try and create a flow which is non-stop (an urban motorway or freeway) - to do this they use flyovers, subways, viaducts, underpasses.
And in the process some simple creative methods are ignored. One being the 'banning of certain right / left turns. This works when - traffic is diverted away from a busy segment to an unused or underused segment. Most people think if they take a bigger arterial road they will get faster to their destination - this is not true (and we will all have experienced this). In the process, the majority try and use the arterial road and saturate it but smaller parallel feeder roads are oft lying vacant/under-used.
The solution then can be like how the banks manage their queues, by removing a certain road, pedestrianising it or creating one way gyratory systems or banning right / left turn to divert some of the vehicles to unused sections - a detour, but overall traffic flow improves, journey becomes reliable and people reach their destinations more quickly (I suspect this also has a positive impact on pollution). Sometimes, this is done by synchronising of signals - a method used is phasing signals - i.e. green at junction one, the next junction becomes green by the time cars reach it and so on. But by phasing, what they mean is that junctions 2,3 and 4 will have incrementally longer green phase to improve throughput. Often when traffic is made to detour on smaller unused road to make the most of available capacity, this raises concerns over pedestrian safety - hence these measures may have to be used with traffic calming methods.
Thus, there is nothing wrong in the method as long as -
Pedestrians are not compromised.
If the overall flow is improved.
However it could all go wrong IF -
- Pedestrians are not given a priority
- Traffic flow will not improve if chaos and congestion is simply to be pushed from one junction to another. In other words a lot of research needs doing with regards traffic counts during peak and non-peak times to implement the above methods successfully.
Dr Adhiraj Joglekar