I have noticed the effect of altitude on my top speed. Every time I've hit around 50 mph, it has been while coming down from a high Alpine pass above 2,000 metres altitude (6,500+ ft). On similar roads at lower altitudes, I'd estimate being 10% slower than when at that sort of altitude. There's a reason why many speed records are set at altitude.

Knowing that I keep my bike well maintained makes me a bit more comfortable riding at high speeds, plus I make sure that I keep a good grip on the bars and concentrate on the road (no sightseeing allowed at high speeds, it's better to stop to do that). Plus. don't forget how much earlier you need to start breaking for an approaching corner when doing 50mph than when doing 30mph - corners can come up REALLY fast if you're not used to doing that type of speed, and you can end up at the turn before having finished your braking - not a good situation.

My max speed on a single bike is 53 mph (85 kph) and 56 mph on the tandem. On the tandem, it's incredibly easy to do 40+, you don't even need that long or steep of a hill. Those speeds feel way more stable on the tandem than on a single bike, so after getting used to doing 40+ on the tandem, I'm way more comfortable to do it even on a single bike (although due to only being 140 lbs, I need ideal conditions to get there), and only speeds getting close to 50mph now make me a little concerned. BTW, to do those speeds on a tandem, you do need a stoker who is comfortable at speed and very trusting - I'm a lucky guy