Roadside funnel bins

Large bins at junctions and service exits. 

https://www.keepscotlandbeautiful.org/news/roadside-litter/roadside-funnel-bins-come-to-scotland/

When these were trialled at Winchester services on the M3 in Hampshire  the amount of rubbish dumped by drivers fell by 25%.

Because people are lazy. Makes it easier to put their litter in an appropriate bin rather than just throwing it anywhere.