Wide lanes, one-way streets, and cleaned-up sides without obstacles encourage driving faster while narrower passes and trees/bushes on the side make you naturally slow down. On a road or street well designed you would drive at the limit even if you didn't know what the limit is because the limit is set to the "natural" speed of the road. This is what I believe is behind the sergeant's thinking.
On residential streets, a very narrow design would be the first thing to do in order to slow down automotive traffic and make the street more pleasant and safe for pedestrians. There are streets where it's nearly impossible to drive faster than 10 mph, so there are really no practical limits for speed governance in road design.
Conversely, on highways even astonishingly high speeds can be reasonably safe such as on German autobahns. Highways are mostly protected against the worst accidents such as head-on-head collisions. Things usually go wrong when someone disrupts the traffic flow by unattentive, blind lane changes. This is a completely different safety assessment than what's in effect on the streets of populated areas.