Humans have been constructing crude bridges from the first time they laid logs across a stream or river they needed to cross. Although a brick arch bridge is said to have existed in Babylon in 1800 BC, most bridges of this time period were probably made of wood. The Romans developed bridge building to a degree that it took Western medieval engineers many years to match. Roman bridges were often composed of several stone arches which supported a flat road. Bridges of this type date back to as early as 219 BC. It wasn't until the mid-1800s that bridge designs began to incorporate metal for added strength and permanence. Early truss bridges used wooden trusses bound with iron tie-rods. By 1850, wooden trusses gave way to steel. Modern bridges incorporate designs ranging from concrete and steel arches to steel girder and suspension styles. The development of modern bridges constructed of durable materials was vital to the expansion of the worlds railroad and highway systems.