Choosing a design speed means finding a balance between several interests which compete for priority,[1] such as high vehicle speeds to allow drivers to travel to their destinations quickly versus low vehicle speeds for the safety of people outside the vehicle (such as pedestrians and cyclists),[2] or quick movement of peak traffic (traffic engineering) versus maximising the economic development potential of the street (urban planning).
Roads with higher design speeds require wider lanes,[2] sweeping curves, wider recovery areas, broader clear zones, steeper curve banking, longer sight distances, and more gentle hill crests and valleys.
[2] Roads and streets with lower design speeds require narrower lanes, sharper/tighter curves, smaller or no clear zones, less banking, less sight distance, and sharper hill crests and valleys.
The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway."
Significant reductions in speed from one segment to the next are flagged as locations where drivers may end up driving too fast for road conditions.