Can a road bike stand constant 40km/h for long distances? I am planning on converting my road bike into an electrical one and I can see that I can get average speeds of 35-40kmh.  So I am planning on biking long distances at this average speed. Is it safe to do it? What can fail at such high speeds in long run, considering different types of asphalt quality. I know that it depends on the bike manufacturer, but can a generic failure be anticipated?
Stopping distance is definitely something to be aware of. As speed increases, the distance needed to stop increases non-linearly. For illustration, in cars, with 1 sec reaction time:


20 km/h = 8m
30 km/h = 13m (50% increase in speed, 63% increase in distance)


(Source: http://www.nzci.co.nz/tools-calculators/stopping-distances.html)

In other words, it may feel safer than it really is.

Also make sure your conversion has a brake sensor cutoff, you wouldn't want your braking distance increasing because the motor is pushing while you're frantically trying to stop.