If Bristol's newest bus service is anything to go by, the future will be various shades of brown.
You may have seen the UK's first poo bus out and about at the end of 2014 when it ran very briefly between Bath and Bristol Airport, but the bio bus – to give it its preferred name – is about to go into regular city-centre service.
From March 25, First West's biomethane-powered bus, fuelled by human and household waste collected from 32,000 homes, will pootle around Bristol on a 15-mile route between Cribbs Causeway and Stockwood.
It might not prompt the sort of celebratory scenes that met the Olympic torch relay a few years ago, but the toilet-themed artwork is sure to raise a smile from people living along the route.
The bus, which seats up to 40 people, will only be able to refuel at a special site in Avonmouth, which makes running the bus a tricky affair compared to the rest of First West's buses. But if all goes to plan when the bus starts running four days per week, the transport company might consider adding more bio buses to its fleet.
And the bus's route number? The number two, of course.