There are rules in place, the main one being................you must not empty directly into roadside drains as they feed directly to rivers and could kill fish and other animals.
The major flaw in this, is that the waste from, car washes / graffiti removal / stone cleaning and other activities runs straight onto roads and down the drains in massively high volumes compared to what a carpet cleaner produces.
Personally...........I've always used a toilet if available, otherwise, over grass or a handy shrubbery.