11.1: Introduction

A road is an identifiable route or path between locations. Nearly all roads are constructed in some way, typically by smoothing the natural landscape and often by paving. Roads serve as transportation routes, accommodating bicyclists, horses, vehicles, and pedestrians. Roads also provide access to property and provide right-of-way to other infrastructure systems such as pipelines or telecommunications cables. Reflecting on the ubiquity and importance of roads, there are many words denoting roadways, including: avenue, boulevard, court, drive, freeway, highway, parkway, street, etc.
Roadway construction dates back over 6,000 years. Early roads were paved with timber, brick and stone. The Roman Empire was noted for an extensive network of paved roadways, covering roughly 78,000 km in Europe and North Africa 2,000 years ago. Figure 11.1.1 shows a modern picture of former Roman road paved with stone and still in use in Syria.

Figure \(\PageIndex{1}\): Example of a Roman Roadway Still Maintained. Source: By Bernard Gagnon (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...ll_Aqibrin.jpg

Roads can be one-way, but most are divided in the center and accommodate two-way traffic. About a third of worldwide roadway traffic follows the convention of vehicles traveling on the left side of roadways, while two-thirds follow a right-side convention, including the United States (Figure 11.1.2).

Figure \(\PageIndex{2}\): Countries by handedness of traffic, c. 2017. Source: By Benjamin D. Esham (bdesham) - Created by bdesham in Inkscape from BlankMap-World6.svg, using information from Sens de circulation.png.This vector image was created with Inkscape. https://commons.wikimedia.org/w/inde...?curid=2653447. Countries driving on the left shown in blue.

Modern pavements are complex systems themselves, with multiple layers and drainage systems. Figure 11.1.3 illustrates a flexible (asphalt) pavement used for rural interstates in Idaho. The pavement has a surface layer of asphalt (six inches of plant mix bituminous pavement), a layer of asphalt treated permeable leveling course (two inches of ATPLC), gravel, a rock cap, a granular subbase, and a subgrade geotextile. The pavement is slanted to facilitate rain run-off.

Figure \(\PageIndex{3}\): Flexible pavement rural interstate, typical section for Idaho. Source: FHWA, Public Domain, https://www.fhwa.dot.gov/publication...21/appende.cfm

Roadways are used for numerous purposes and by multiple types of vehicles. Table 11.1 shows vehicle miles travelled (in millions) for different types of vehicles and types of roadways. Limited access interstate roadways have significant amounts of traffic, but most traffic is carried on other types of roadways. Motorcycles, buses and truck traffic are significantly smaller than light duty vehicle traffic.

Table 11.1.1: U.S. Vehicle Miles Traveled by Type of Vehicle and Type of Roadway 2014.

Roadway Type	Light-Duty Vehicle (million miles)	Motorcycles (million miles)	Buses (million miles)	Trucks (million miles)	All Vehicles (million miles)
Rural Interstate	173,000	1,000	2,000	56,000	231,000
Other Rural	505,000	6,000	4,000	76,000	690,000
Urban Interstate	458,000	2,000	2,000	57,000	520,000
Other Urban	1,476,000	11,000	8,000	90,000	1,585,000
Total	2,711,000	20,000	10,000	279,000	3,026,000

Source: FHWA, Public Domain, Highway Statistics, https://www.fhwa.dot.gov/policyinfor...s/2014/vm1.cfm