Sustainable vs Potentially Sustainable

Buses are advertised and declared to be “Sustainable”. However, the simple presence of buses on urban roads doesn’t necessarily reduce transportation’s emissions or traffic congestion in the area. In some cases it achieves the opposite.

We already demonstrated that for several decades to come, the energy for mobility will come from fossil fuels, no matter whether we have a tailpipe at the back of our vehicles or a green rectangle on the reg plate. The main factor that determines the energy consumption is the vehicle mass.

If we compare the displaced mass/passenger of a car vs a bus, as a function of occupancy, we get this representation:

If a double-decker bus has 31 passengers the weight/passenger transported by it from A to B is the same as that of a Sedan car with 5 occupants. From 32 passengers up the bus becomes more efficient than a car.

If the bus occupancy drops below 6 passengers it becomes less energy-efficient than a single-occupancy car, and its inefficiency goes through the roof if the occupancy drops further. While during peak hours in congested cities, one might encounter buses with over 31 passengers, during off-peaks most of them are rather nearly empty, and towards the end of the shift they are single-occupancy displaying “Not in Service”, wasting an enormous amount of energy/emissions.

The average bus occupancy for Great Britain is displayed in the table below, making them be the same efficient as cars transporting 2 people.

Yes, I know there’s a sound reasoning behind keeping them in motion. We need to let people know that buses are there and can be relied on, and maybe someday they will ditch their cars and become bus enthusiasts, and urban transportation will finally become sustainable. Well, that didn’t happen for the last 70 years. Why should we expect it to happen from now on?

One might say that the bigger the engine/motor the more efficient, and I’m not going to argue on that. However, the energy use peaks when vehicles accelerate, and buses do that quite a lot every time they leave a bus stop. Not to mention that I haven’t seen a single bus in the UK stopping its engine while passengers board/alight. Fortunately, electric buses eliminate the idling issue.

So, three more factors need to be met for buses to be truly sustainable:

1. To increase the bus occupancy in peak hours to 50-60 to compensate for the inefficiency during off-peaks and get an average of over 32 passengers.
2. To have enough buses to accommodate the majority of car users.
3. To persuade car users to give up their well-being standards and choose a less comfortable, more complicated and time-consuming commute.

Until they meet these criteria buses remain only POTENTIALLY SUSTAINABLE from an energy/emission perspective. However, there’s a “meeting needs” facet to sustainability and that will be even more challenging to achieve.