afaik most ebikes are limited at 20mph, and realistically the thing that kills most cyclists is cars not the speed of cycling. And when a car runs you over a styrofoam helmet isn't going to help much.
Even if not limited in speed, the assist is limited so on flat ground you are still limited to under 30mph. However downhill any bike can get going much faster - I've been to 35mph on my non-electric bike (my electric bike is slower downhill, but there difference isn't much), and I live in an area known for how flat it is - those of you are live where there are real hills (much less mountains) can easially get going much faster.
Hit 45mph heading down Tioga Pass leaving Yosemite headed east. With a loaded BOB trailer[0]. Could've got going a hell of a lot faster, but I thought the better of it.
35 is hauling pretty good ass on a bike. It's not a speed it'd be advisable for someone to just jump on a e-bike and do. Fortunately, most e-bikes made by reputable manufacturers top out at 20 or 28. Unfortunately, there are plenty of less reputable brands. Not that nobody can ride faster safely, but this is a case where earning your speed will give you the experience to manage it safely.
[0] Before they pivoted to the larger and more lucrative stroller market, they made excellent single wheel bicycle trailers for touring.
