How Did Roads Develop
Roads have been important transport systems for many cultures. While we take their place and presence for granted, as something universal in landscapes today, the reality is several factors often develop before these features become common. Furthermore, roads often reflect levels of authority that begins to reflect how power is distributed in society and where it ultimately is found.
Early Development of Roads
Paths used by travelers likely developed in response to physical or geographic obstacles that made movement slow or difficult if one did not use a given road. However, with the innovation and spread of agriculture, roads in the countryside began to be placed in lands between fields. In other words, roads soon were created to control traffic between settlements and to access regions around settlements. Roads became ways in which people could be controlled so that their movements would not disrupt agricultural activity or even private property, as the concept of property itself began to create the idea that roads were common space that helped avoid and access private spaces. With agricultural cycles in the Neolithic becoming more common and fixed on the landscape, roads became more fixed features since they proved to be expected ways that one can move without disrupting agricultural activities. Regions that developed pastoral-based economies, on the other had, may have not developed fixed roads, as movement did not need to be regulated or confined to specific spaces. 
Roads developed similar in towns and cities. As private areas became established, roads were utilized as a means to access or avoid private regions in cities all together. At first, roads were not paved, but by the 4th millennium BCE, paving was already developing in the Indus region using baked bricks. Concepts of lane use likely developed as wheeled and human traffic took shape by the 4th millennium BCE if not earlier. Donkeys and onager were likely the most common form of animal in early urban streets to be used as transport. The horse, domesticated in Central Asia, likely did not become prominent on urban streets until the 2nd millennium BCE.
Because roads developed as common space for access that also regulated flow through a city, gateways became important areas that developed to control traffic coming in and out of a city. As cities grew in wealth and importance throughout the 3rd millennium BCE in the Near East and Indus, both these regions likely developed city walls that could only be breached using official roads via gateways. This helped cities also use these control points as a way to tax trade or regulate what was coming in and out of cities.
The next major change to roads was the development of the "royal roads" or official government roads that connected very distant towns. This idea developed in the Middle Assyrian and Neo-Assyrian period, from the late 2nd millennium BCE and early 1st millennium BCE. These roads were roads the government had authority over and to utilize them one had to have official permission, unlike normal roads that were considered as common space. The idea of these roads was for developing rapid transport to connect distant places of an empire or large state for military purposes. Unlike many early roads connecting towns, these long-distance royal roads developed to be more linear or straight, as speed was the primary goal of these roads. Such roads became particularly important as horses developed as the primary transport option for military purposes as well as messengers. Royal roads also helped develop the concept of highways, where a type of road would bypass local roads and help those moving on these roads to more quickly move without going through each city or town along the way. Major nodes or towns, rather than small cities or towns, became the focus in royal roads. Inns and rest places developed as such long-distance roads became important features for large empires. In effect, unlike earlier roads, the growing state needed to create systems of communication that was rapid. Royal roads became the chief vehicle for this along with horses.
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. 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. 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.
In the Islamic world, innovations were developed in the types of pavement used. Baghdad developed the use of tar that derived from pitch or bitumen. In fact, this was already present in ancient Babylonian cities in ancient periods. The use of bitumen as a type of tar was a forerunner of tar and asphalt roads that are today widely used. Bitumen was relatively easily available and its waterproofing qualities meant that streets could become relatively dry quickly and remain mud free.
By the 18th century, road technologies once again began to improve as engineering was more greatly utilized. Concepts of drainage now led to roads being created that were developed so that water can run to the sides and be drained in areas away from the main central road to keep roads dry. Specific stones now were utilized for their qualities, such as thickness and durability. Road maintainers were employed to keep sections of roads well maintained throughout the year. By the early 19th century, road paving using a combination of dirt and stone proved to be useful for road durability, leading it to be a common combination. Such types of roads remained common when in 1901 tarmac was, for the first time, utilized to pave roads. The idea was similar to what was used in the Medieval Islamic period; however, tar was mixed with aggregate material to create a more mixed product than pure tar from bitumen. This mixture became the type of surface we today see in most roads and has developed as the chief road surface.
Roads developed due to social needs to communicate and connect with people. However, roads also developed to keep people away from land used for agriculture while also regulating where people can go. Road technologies and design improved in antiquity, as rapid movement involving soldiers and the army became an advantage to growing empires. Technologies for roads did not improve significantly in many places after the fall of the Roman Empire. It took 18th century industrialization to prompt new types of roads to be built that improved drainage and road surface. By the early 20th century, the use of tar mixed with aggregate led to the development of tarmac and asphalt used for modern roads today.
- For more on how or why roads first developed, see: Alcock, S. E., Bodel, J. P., & Talbert, R. J. A. (Eds.). (2012). Highways, byways, and road systems in the pre-modern world. Chichester, West Sussex ; New York: Wiley-Blackwell.
- For more on early transport, see: Sherman, D. M. (2002). Tending animals in the global village: a guide to international veterinary medicine. Philadelphia: Lippincott Williams & Wilkins.
- For more on taxing of road transport and movement, see: Nail, T. (2016). Theory of the border. Oxford ; New York: Oxford University Press, pg. 86.
- For evidence of such royal roads, see: Altaweel, Mark; 2003. "The roads of Ashur and Nineveh," Akkadica 124: 205-212.
- For more on the infrastructure in relation to royal roads, see: Briant, P. (2002). From Cyrus to Alexander: a history of the Persian Empire. Winona Lake, IN: Eisenbrauns, pg. 364.
- For more on how long-distance roads benefited trade, see: Bakhtia, L. M., & Bariand, P. (2011). Afghanistan’s blue treasure lapis lazuli.
- For more on Roman engineered roads, see: Nardo, D. (2015). Roman roads and aqueducts. San Diego, CA: ReferencePoint Press.
- For more on Islamic roads, see: Bobrick, B. (2012). The caliph’s splendor: Islam and the West in the golden age of Baghdad (1st Simon & Schuster hardcover ed). New York: Simon & Schuster.
- For more on how early industrialization improved roads, see: Guildi, E.J. 2001. The Road to Rule: The Expansion of the British Road Network, 1726--1848. Proquest.
- For more on the development of tarmac and asphalt for roads, see: Ralston, A. (2005). Between a rock and a hard place. London: Pocket.