We rode around Tibet for three months before the cops nabbed us and they couldn't have been more courteous or helpful. This is not always the case and the further south and west (towards Lhasa) you go, the more hardline are the cops you'll encounter. Some towns are definitely not places to linger, eg, Qamdo, Zedang, Bayi, and apparently Zogang but no longer Markham, and in Yushu the police are vigilant but fair. Permits are required to travel around (but not in) Lhasa, and in Lhasa it is not possible to get visa extensions or permits unless you are on an authorised tour. Permits are not required to ride the main road from Lhasa to Zhangmu (don't know why anyone would want to ride this road though) unless you want to visit sights along the way like Everest, Sakya or Tashilumpo, and these can be acquired easily in Shigatse  A caution - regulations change; fines are at the discretion of the police officer.