One fitting new road type could be bike lanes, which could be used by couriers on bikes or by NPCs. (In addition to asphalt as standard).
A new road type could be cobblestone streets, which could be used to create old-town areas. (In addition to asphalt as standard).
It would be great to control traffic from couriers (or maybe one day, NPC cars). One option could be one-way streets. (In addition to asphalt as standard).
Add a road type parking zone where couriers (and maybe one day, NPC cars) can park in addition to the regular parking spot. (In addition to asphalt as standard).
Bike lanes would be great