Can a road bike stand constant 40km/h for long distances?
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.