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.
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).
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.
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