Does driving speed affect alertness?

It is sometimes claimed that a lower speed (limit) adversely affects alertness and, thus, road safety. We are, however, not aware of any studies that have researched this. What we do know is that having too few or too simple tasks for an extended length of time (when work load is too low) may lead to fatigue which adversely affects road safety (see SWOV fact sheet Fatigue). In addition, changing driving speed proves to have a positive effect on alertness [21]. Yet, all research points out that lower speed has a positive net effect on road safety: if average speed on a particular road is reduced, the number of crashes decreases (see the question How great is the effect of speed on road safety?).