Police checks take place on travellers throughout the country. They come in three forms. One is a check on local transport; European cars, or rental cars, are usually waved through. Buses (other than CTM services) are much more likely to be stopped, but usually only briefly. The second kind of police check is a routine but simple passport check - most often polite and friendly, with the only delay due to a desire to relieve boredom with a chat.
The third kind of check is more prolonged and involves being stopped by police stationed at more or less permanent points on the roads, who will conduct a fairly detailed inquisition into all nonresident travellers. There is a considerable amount of form-filling and delay. This occurs most commonly entering the major towns of the Western Sahara (Laayoune, Smara, Boujdour and Dakhla), but such checks are diminishing in number and are really now just a formality as the situation gradually becomes more and more stable.
In the Rif mountains , especially around the cannabis-producing region of Ketama, you may also come across police checks - concerned, obviously enough, with just the one substance. Buses are usually delayed more than grands taxis at such checkpoints. There are also several and sometimes lengthy checks for duty-free contraband on buses from Nador to Fes.