Sometimes, two buses of the same service arrive at the same time with one bus being overcrowded and the other almost empty. Why?
The reason for this is largely due to adverse traffic conditions on the road and other conditions such as road closure, diversions, weather, accidents etc. Indeed, more than 70% of our delays or bunching are due to these reasons.
For example, assuming 3 buses started out being 10min apart which was sufficient to cater to the demand along the route (see below).
Bus 1 did not meet any problems and hence, was able to run on schedule. Bus 2 encountered a jam while Bus 3 did not encounter any problems. Hence, the time between Bus 1 and Bus 2 was lengthened, while the time between Bus 2 and Bus 3 was shortened (see below).
By taking more than its share of passengers Bus 2 slowed down as a result, while Bus 3 picked up fewer passengers which caused it to speed up. Bus 2 was thus caught in a vicious cycle that came to a head when the third bus caught up.
To the passenger on the ground, he would have waited 15min and it would seem that Bus 2 was crowded while Bus 3 was relatively empty. In the above situation, adding more buses is not a solution, as the same buses will end up being caught in the jam. More bunching will occur when they do eventually emerge from the traffic slowdown.
We assure our commuters that we try as far as possible to ensure that our buses arrive at regular intervals including adjusting the assignment of buses to suit real time conditions though there is a limit to how changes can be made.