as a greater proportion became more regular in nature, such as standard daily loads of equally-sized boxes from a distribution centre to a supermarket, rather than the unpredictable ad hoc nature of earlier road transport. Flatbeds are still in use, but are now used for more specialised cargoes, such as constructional steelwork or lighter abnormal loads, such as machinery, lumber loads/dry wall or any load that requires use of a forklift without the use of a loading dock. Low loaders, for construction machinery and heavy plant vehicles, are not considered as flatbeds. Neither are abnormal load carriers for heavy haulage. Configuration/design in US trailers In North America, the length is commonly , and the width is either (including rub rails and stake pockets on the sides, which generally placed every ). Some older trailers still in service are only or shorter if used in sets of doubles or triples (often used to haul hay). Various lengths and combination setups can only be legally driven on turnpike/toll roads which are far too long for most roadways. Body and frame can be one of three general designs: the heaviest and sturdiest is all steel (usually with wood planks), ever-popular combo with steel frame and aluminium bed, these type often have wood portions for nailing down dunnage boards), and aluminium (which is the lightest allowing for more cargo to be legally carried without overweight permits). Incredibly light and very expensive to purchase, all aluminium trailers are very slippery when wet, flex more and are easily damaged. They also have a natural upwards bend so that when loaded they straighten out to be flatter, rather than to sag in the middle under a load. Another popular type of flatbed trailer is a step deck (or drop deck) with approximately 2 feet lower deck and low profile wheels to accommodate taller loads, without hitting low bridges or tunnels. These stepdecks can come with loading ramps to allow vehicles to roll on and off of the back from ground level. Shorter trailers used for local jobs such as landscaping and building material delivery within urban or local areas can have a "hitchhiker" type forklift truck attached to the back in order for driver alone to deliver and unload pallet/skid items. A bulkhead or "headache rack" is sometimes attached to the front of either a straight or a stepdeck trailer for load securement at the front of