Just to put the record straight on this, Franklin does not say that you should never undertake (at least in the verson of his book Cyclecraft that I possess). He warns specifically that you should not undertake cars moving faster than walking pace and to watch out for car drivers signalling to an approaching vehicle to turn right across their path. He does also say that you should only undertake large vehicles when they are stationary, and not if you believe they may start moving.And why should John Franklin know any better? .
Chris