Originally Posted by
mr_bill
18 inch curb isn't so much a "concrete barrier" - it's a tall curb.
It is quite common for there to be a standard curb, but then include additional deterrence such as bollards, additional segmented curbs, or planters/plantings between general traffic lanes and the bikelane.
Some of those are narrow and twisty (especially, at intersections). This type works for slow cycling.