The EU fuel economy drive cycle does involve driving on a rolling road, but this includes some hills and accelerating from rest a couple of times. However, this is done without air con, with tyre pressures at the set limits (and sometimes on the lowest rolling resistance tyre option), with correctly set loadings for an unloaded vehicle without options (the spare wheel is now an option on many cars for this reason) (take all the crap out of your boot for a start), and other settings optimised. The manufacturer cannot publish figures that cannot be achieved, and this has to be certified by the VCA, however, on the open road, you will have to drive VERY conservatively to achieve them (and forget taking passengers, a boot full of gear, bike on the back etc). Most people will get somewhere near (to within 10-15%), but only a small handfull will get there. Bear in mind that even driving into a headwind will have an effect.
And I agree, a Prius is false economy on normal roads – it is just lugging round heavy batteries and motors and never using them. Only driven by people who are “environmentally friendly” (and if you told them that digging holes in your back garden would save the planet, they would dig holes….)