I struggle to reach higher heart rates on the turbo, whereas I can easily push up into the 90% (albeit for very short periods) on the road. In fact if targeting a particular HR I have to keep it down if on the road and keep it up when on the turbo. I wonder if partly it's because there's a heaviler 'baseload' of balancing and steering and not getting run over activity when on the road, as well as the adrenalin fuelled/rush of blood to the head/imagining you wiggo/cav/gee, ill-advised efforts to impress girls/show drivers your serious/win SCR points effect.