I don't know much about how Cambridge's P&Rs are set up. Logically some P&R bus services could do a tour of peripheral suburbs and nearby villages before calling at a major car park location and going limited stop into town. Most I've experienced don't do this, instead operating as a simple shuttle between car park and urban centre instead. If a P&R is subsidised, extending (some of) its bus operations into the surrounding suburbs could be considered unfair competition against any established commercial bus operations, and we've also got the issue that unitary authorities often have travel to work and play residential districts extending well beyond their limited political boundaries, but can't spend subsidy for any improvements physically in an adjacent local authority, and if the political hue over the border is historically very different, best of luck to them formulating any holistic joint approach to such transport planning. Of course the P&R buses DO compete with other local services anyway as some people soon find it more convenient to drive to the frequent and fast P&R bus rather than wait for the sparse and slow normal one. P&R alone can thus tend to reinforce auto-dependency in these cases, especially if the local bus services are then subject to service cuts due to falling patronage. Modern light rail schemes so far seem to have been able to successfully address both these roles on the same line without outcry and there's no reason a largely segregated rubber-tyred transit mode couldn't do the same, if planned in the same way and delivered reliably. It seems sensible to stick with existing technology already in use locally, so perhaps what's needed in Cambridge is a fast reliable route between the existing and future busway segments and extensions running quickly and directly through the city centre and making a limited number of strategic calls. Problem with that is the busways are used predominantly by longer distance routes using diesel buses. It would seem odd if after rushing headlong through the countryside to the peripheries, these guided vehicles would have to turn off onto the regular roads while this new city centre core tunnels played host solely to inner-city, all-electric, rubber-tyred tram analogues on urban routes alone. These could be hybrid I suppose, with electric operation mandated on the city core routes, whether they turn out to be in (at least in some parts) tunneled or on the surface. For low speed street / segregated operations in the centre, the mechanical guidance systems employed would not be suitble due to the raised curb required. Buses on these routes could also be fitted with an alternative guidance system based on self driving technology paving the way towards future more comprehensive automation.