The average lowest underpass on most developed roadways in urban areas allows for around 14' of vehicle height. If the height of the flatbed is around 3'-4' and your van sits on top at around 9', you're getting close to losing your roof mounted stuff......
Interstate Highway Standards (2007)
Minimum vertical clearance
under overhead structures (including over the paved shoulders) of 16 feet (4.9 m) in rural areas and 14 feet (4.3 m) in urban areas
, with allowance for extra layers of pavement
. Through urban areas at least one routing should have 16-foot (4.9 m) clearances. Sign supports and pedestrian overpasses must be at least 17 feet (5.2 m) above the road, except on urban routes with lesser clearance, where they should be at least 1 foot (30 cm) higher than other objects."