At the risk of sounding insensitive I’ll say this: most of the plastic stuff that eventually clogs these drains doesn’t just find its way there. It’s thrown around carelessly by otherwise normal and perfectly sane human beings, who then act surprised when the inevitable happens. I find it difficult to sympathize with such, honestly. Bad & unfortunate as the situation might have been.
Perhaps the ‘nyumba kumi’ initiative wasn’t such a bad idea after all.
I think you might be closer to the solution then you think, however I doubt that banning private cars is the solution, a trick however would be to setup a system where if you want to enter/pass through the city you have to pay a amount of money, of course different depending on the time of day (rush hour would be higher and none-rush hour would be lower). This would ensure that people who really need to get to or through town are able and also to minimize traffic while in town, in addition it becomes a income generation that can be used to construct highways around town that can be used by everyone else that just want to get back home and not go into cbd.