Mountable curbs have been suggested as a way to separate bike lanes or so-called “cycle tracks” from the rest of the street.
I am not in favor of such curbs. After all, any obstacle has hazard potential, and a mountable curb can have more than you would imagine.
As a cyclist rides along a mountable curb, tire “drift”, the slope and the offset front-tire contact patch make the bicycle hard to steer, so it must be held in a straight line by a continual effort. A similar issue with a different cause is discussed here.
Worse, if a mountable curb is slippery, it is no longer mountable, though it may deceptively still appear to be. The resulting fall may be a skidding-type fall or a diversion-type fall, in which the bicycle’s front wheel is swept to the side.
Trash, sand, snow or ice can accumulate along the curb. Installation of a curb-separated “cycle track” on a conventionally crowned roadway would require sloping the road surface away from the new curb to avoid this problem.
Even this unusual measure would not make snowplowing or street cleaning any easier, with an abrupt longitudinal change in elevation in the traveled way.
An approach used in Copenhagen is described here — a second line of drains at the curb (in the case shown, not mountable). The drains carry away standing water but does not solve the problems with trash, sand, refreezing water in the added, new gutter, snow clearance or street cleaning.
Cambridge, Massachusetts bicycle coordinator Cara Seiderman has shown a photo of a machine with a large, cylindrical, horizontal rotating brush used to clear snow from Copenhagen facilities. Perhaps this answers some of the issues with snow clearance and perhaps it also is used for street cleaning, but the expense is considerable, and the need for such equipment may not even be considered when a facility is constructed.
Snow clearance and ice melt issues on the 9th Avenue facility in New York City are described here; this is a barrier-separated rather than curb-separated facility, but the issues are similar. The bikeway here is wide enough that conventional snowplows could be used, but refreezing of meltwater from the windrow of snow between the bikeway and the street required heavy salting, and salt rusts bicycles.
So, is the prohibition of a barrier between a bike lane and the rest of the street in the AASHTO bike guide arbitrary? Does it only reflect issues with cyclists’ being able to enter and leave a bike lane, which can be addressed with a mountable curb? Not hardly.
Promotion of curb-separated bikeways, without considering the technical issues,or with “band” aid solutions such as mountable curbs, is all too typical of much bicycling advocacy.