Tidal Flow Operations
Look at the following timetable:
Service No 2 Town Centre – Goodwin
| Bus No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 1 | 2 | 3 | 4 | 5 |
| Town | 0700 | 0705 | 0710 | 0715 | 0720 | 0725 | 0730 | 0735 | 0740 | 0745 | 0750 | 0755 | 0800 |
| Goodwin | 0716 | 0721 | 0726 | 0731 | 0736 | 0741 | 0746 | 0751 | 0756 | 0801 | 0806 | 0811 | 0816 |
| Goodwin | 0718 | 0723 | 0728 | 0733 | 0738 | 0743 | 0748 | 0753 | 0758 | 0803 | 0808 | 0813 | 0818 |
| Town | 0734 | 0739 | 0744 | 0749 | 0754 | 0759 | 0804 | 0809 | 0814 | 0819 | 0824 | 0829 | 0834 |
In the peaks passenger movements are predominantly in one direction, and usually headways are the same in each direction only because of the need to get buses back to the end of the route in order to carry out the next ‘in service’ trip.
It’s assumed for the purpose of this exercise that demand on Service No 2 is greater into the Town Centre (i.e. the morning peak) and that the 5 minute headway in both directions is for positioning purposes only. The opposite would be the case in the evening peak.
Using ‘dead’ running to achieve tidal flow operation and save a bus
The principle of tidal flow operation is that buses operate ‘in service’ in one direction, and ‘out of service’ (dead) in the opposite direction. Naturally, ‘dead’ running time must be less than ‘in service’ running time. For the purpose of this exercise, a dead running time of two-thirds service running time is used.
Before this can be done, it’s useful to look at the calculations that brought about the Working Timetable shown above.
16 + 16 + 2 + 2 / 5 = 36 / 5 = 8 buses + 4 minutes
The level of service is going to be changed to provide headways:
- every 10 minutes in both directions
- every 5 minutes in the direction of the peak flow
This will result in a 10 minutes headway for buses competing a round trip in the full round journey time:
16 + 16 + 2 + 2 / 10 = 36 / 10 = 4 buses + 4 minutes
and a 10 minutes headway for buses completing a round trip in less than the full round journey time. The formula for the ‘dead’ running element is as follows:
Live running in one direction + ⅔ running in one direction / headway
= number of buses
16 + 12 + 2 / 10 = 30 / 10 = 3 buses
The Working Timetable now looks like this, with one bus saved:
| Bus No. | 1 | 5 | 2 | 6 | 3 | 7 | 4 | 5 | 1 | 6 | 2 | 7 | 3 |
| Town | 0700 | 0710 | 0720 | 0730 | 0740 | 0750 | 0800 | ||||||
| Goodwin | 0716 | 0726 | 0736 | 0746 | 0756 | 0806 | 0816 | ||||||
| Goodwin | 0718 | 0723 | 0728 | 0733 | 0738 | 0743 | 0748 | 0753 | 0758 | 0803 | 0808 | 0813 | 0818 |
| Town | 0734 | 0739 | 0744 | 0749 | 0754 | 0759 | 0804 | 0809 | 0814 | 0819 | 0824 | 0829 | 0834 |
Bus workings 5 – 7 operate in service from Goodwin to Town Centre returning ‘dead’ after each trip
The TransACT Blog 