Okay....another work week over with. Tomorrow I'll work some more on my new bike. But tonight I can drool over some of the bikes I see in this forum and even write a bit.
Concerning speeds, I strongly recommend not going anywhere near 30 mph. Most bicycles were not engineered for the shocks and stresses that they'll experience at such speeds. This doesn't mean that going 30 means a definite, instant death. But the margin of error is too small and the odds are getting short enough that there's a measurable chance of them catching up to you in fairly short order. People could bet money against you and be reasonably certain of getting a payoff.
Imagine yourself on a plain old pedal bicycle. You've got smooth pavement and a light tailwind. Not enough that the wind is doing most of the work for you. But just enough that you can keep up a nice pace without wearing yourself out. Enough that you find yourself thinking, "Hmm....I'm doing pretty good here. If bicycling was always this easy, I'd be doing it all the time".
We're talking 13 to 15 mph here. That's about how fast I ride a china girl. 95% of the time, anyway. I'm willing to goose it in a few special circumstances. But never for very long. When the road opens up for me again, then I slow it down again.
That might not sound very fast. But it's faster than you might think because you can do it all the time. Headwinds and uphills make no difference. And that's huge.
I'll use my own work commute as an example. It's about 6 miles door to door. Leaving home for work I climb a fair amount and then I go downhill a good amount. Work is at a lower elevation than home.
On a regular pedal bike, if the wind is not against me, I make the trip to work in about 35 minutes. On the way home it's about 45 minutes. But that's if the wind is not against me. But the direction is more or less southwest and 3 nights out of 4 the wind is definitely against me. So my realistic home time is closer to 1 hour.
On a motored bike my time in each direction is about 30 mins. That motor equalizes the uphills and the headwinds for me. And it does more than that. I manage this with less effort and sweat than I do on a pedal bike. We all know this already, of course. But it means that what we might call me 'effective speed', taking effort into account, at 15 mph on a motored bike is more than double my 'effective speed' on a pedal bike.
To put it more succinctly: if you think of your machine as an ultra-capable bicycle, then you'll do well and you will be pleased with your arrival times at your various destinations. If you think of your machine as a light and inexpensive motorcycle, then you'll have somewhat better arrival times. But it'll be at the cost of a much greater exposure to the possibility of death or maiming. And your arrival times might not even be all that much better simply because of 'friction' with other vehicles (cars) on the roads.
I consider 20 mph to be going too fast. No one has to go slower than that simply because I say so. But I'd beg anyone considering the subject (particularly newbies) to think about it. We don't want to lose you.