Changes

Although royal roads proved to be useful for military purposes, as armies after the Assyrians began to adopt such roads, it also led to the development of long-distance trade along secure roadways. In part, long-distance roads helped to spawn the developing Silk Road connecting trade between China and Europe (Figure 1). The long-term legacy of royal roads that allowed armies to move quickly were later adopted by various armies.<ref>For more on how long-distance roads benefited trade, see: Bakhtia, L. M., & Bariand, P. (2011). Afghanistan’s blue treasure lapis lazuli.</ref> The modern highway system created in the United States and other countries reflect the concept of rapid movement along large distances by the military, showing that the concepts born in the Iron Age continued to be adopted even by more modern transport. In effect, highways were always seen as initially being for the military above all, but benefits of rapid movement also led to trade thriving along routes, similar to highways today.

==Improvements in Paved Roads==

While highways and royal roads developed by the late 2nd and early 1st millennium BCE, such roads were not paved and often were little more than dirt pathways serviced by rest stations, stables, and inns. Such roads became difficult to travel in wet conditions, making them less than ideal during various parts of the year. In the Roman Period, road engineering reached a new level. Now, roads were built with deeper foundations and underlain with crushed gravel. This helped to drain roads as water could go through rather than be trapped in the clay (Figure 2). Additionally, paved roads were developed on the most important highways.<ref>For more on Roman engineered roads, see: Nardo, D. (2015). Roman roads and aqueducts. San Diego, CA: ReferencePoint Press.</ref> This included using large capstones for pavement. Such pavement gave roads additional speed since wheeled carts and chariots could move more quickly in pavement than on dirt roads. Sometimes these roads were construct with multiple layers underneath the pavement to assist with drainage and strengthening the road. Some of the Roman roads are still used or are visible today. Road technology in Europe, in fact, did not improve to any great extent from Roman designs until about the 18th century CE.