In preparation for my journey to Cambridge, I did review all of the tips on VT's Cambridge travel page. The "warning or danger" category only had two entries: punting and rising bollards. I had to read more about both as I thought punting is what the Dallas Cowboys do on fourth down and had absolutely no idea what a rising bollard is.
Rising bollards are a booby trap barrier designed to destroy trespassing vehicles. There are certain zones of Cambridge which are closed to all automobile traffic, except for buses and taxis, at designated times of day. The borders of these zones are marked by signs that say nothing but "Rising Bollards" beneath an exclamation point. Any car or truck that obliviously or willfully drives into the restricted zone risks punishment from the 'bollards' (metal poles about 3 feet high) which rise up out of holes in the road and impale the invading vehicle like a Middles Ages knight jousting vertically from beneath the ground.
Ordinary tourists need not concern themselves with the rising bollard warning, unless driving into Cambridge with a rental car. That leaves punting (the flat-bottomed boat tours on the River Cam) as the only real danger at Cambridge, as you can still get jousted by some drunk punter's pole.
I almost put this in the "Must See" catagory, simply because if you don't know what a rising bollard is, well, you just have to see it.
These concrete barriers rise up from the street to block ordinary traffic from getting through. Only buses and taxis are allowed access in parts of the Cambridge City Centre, except, of course, pedestrians and cyclists.
There is a special remote tag that will signal the bollard to descend back into a hole, only allowing one vehicle to pass through.
Anyone attempting to follow through will fail miserably, and besides costing them a fortune in fines and repair, they will be the cause of serious traffic delays, as they will be blocking access to the buses and taxis scheduled to run that particular route.
Besides the obvious "Walk" sign, a signal sounding a bit like an alarm clock alerts pedestrians when it is safe to cross the street. When you hear it, RUN! (or get yourself across the street quickly) This is no time for a leisurely stroll.
Buses travel as if they are on a time schedule (imagine that!), and if the bollard is down, they waste no time traveling through it, so please, get out of the way! :-)
If driving in Cambridge city centre DO NOT try and follow a bus or taxi over the rising bollards.
They are designed to keep traffic out of the city centre and only allow permitted vehicle through.
They will come up under your car and cause considerable damage. You will also receive a bill from the City Council for repairing them. It is very unlikely that you will be able to claim for either of these on your insurance.