Just because someone is pulling a van along below the speed limit doesn't necessarily equate to Him/her Not being a competent driver.
Many vanners drive at a slower speed to firstly save on their fuel bill and secondly to save the wear and tear on their car and van.
Flashing lights and hooting the horn at someone that "CHOOSES" to travel below the speed limit is just rude and ignorant and smacks of a driver with a supreme sense of self entitlement, The speed limit is exactly what it states a "Limit" and is Not a target.

