Road

I

Introduction

Road, path established over land for the passage of vehicles, people, and animals. Roads provide dependable pathways for moving people and goods from one place to another. They range in quality from dirt paths to concrete-paved multilane highways.

Roads are used by various forms of transportation, such as trucks, automobiles, buses, motorcycles, and bicycles. Roads allow trucks to move goods from points of production, such as fields and factories, directly to markets and shopping centers. Private individuals rely on roads for safe and efficient automobile, motorcycle, and bicycle travel. Fire departments, medical services, and other government agencies depend on an organized system of roads to provide emergency services to the public in times of need.

The earliest roads evolved from animal paths and served as trails for early hunters. Paths eventually grew around primitive settlements, and as trade grew, longer routes were developed to transport food and other important materials. The use of wheeled vehicles encouraged construction of better roads. The roads built by the ancient Romans were carefully planned and solidly constructed.

Modern methods of road construction were first developed in the 18th century. Innovations of the time included waterproof surfaces and better drainage systems. Modern engineers make use of a variety of materials and construction techniques to build roads that can handle the high volumes and stresses of modern automobile and truck traffic.

---

---

II

Types of Roads

There are many different types of roads, from multilane freeways and expressways to two-way country roads. One important quality of a road is known as control of access. This term describes how vehicles are allowed to enter and exit a road. By controlling access to a road, the road can support more traffic at higher speeds. Roads can be classified into three broad categories: highways, urban or city streets, and rural roads. Each type of road controls access to different degrees. Each type also differs in location, the amount of traffic it can safely support, and the speed at which traffic can safely travel.

A

Highways

Highways are high-speed roads designed to connect major cities. There are many different types of highways. Highways differ primarily in the amount of access control they have and, therefore, in the amount of traffic they are designed to carry.

Highways with fully controlled access can handle the most traffic and are built to the highest construction standards. Interstate highways, freeways, and expressways are examples of fully controlled-access highways. Vehicles that enter or exit these types of highways can do so only at certain points along the highway, generally by using special entrance and exit ramps. The ramps allow vehicles to access the road without disturbing the flow of traffic. Incoming vehicles must merge with flowing traffic, and vehicles leaving the highway use exit ramps that guide them off the highway without blocking the traffic behind. Intersections with other roads are avoided by using either bridges known as overpasses to carry one roadway over another or short tunnel-like structures called underpasses to carry one roadway under another. Finished strips called shoulders on the edges of highways allow drivers of disabled vehicles to make repairs or await assistance without blocking traffic.

Highways with fully controlled access generally have two or more lanes for each direction of travel and often include medians (dividers in the middle of the road) to separate traffic moving in opposite directions. In Europe, highways with fully controlled access are called motorways, motor-routes, or autobahns.

Some highways offer only partial control of access. These types of highways handle less traffic than do highways with fully controlled access. Highways with partially controlled access may intersect other roads at the same level (called at-grade), rather than using overpasses or underpasses. Vehicles can enter highways with partially controlled access at intersections rather than using ramps. However, the right-of-way is often given to one direction of travel, rather than requiring all traffic to stop at the intersection (see Traffic Control). Giving the right-of-way to one direction of traffic helps keep traffic moving at higher speeds, although typically not at speeds as high as those on a highway with fully controlled access. One benefit of highways with partially controlled access is that they are much cheaper to construct than highways with fully controlled access. Many U.S. and state highways are roads with partially controlled access.

B

Urban Streets

Urban streets, which cover cities, towns, and most suburbs, allow vehicles to access properties such as homes and businesses. Urban streets are used by private motor vehicles, public transportation, bicycle traffic, and pedestrians. Urban streets also accommodate underground public-utility facilities, such as electrical wiring, water and sewage pipes, and telecommunications lines. In addition, these streets must often be built around existing buildings and other barriers, such as parks and rivers. Rather than shoulders, urban streets usually have raised edges called curbs, which provide a barrier between the street and the adjoining property or sidewalk.

Urban streets are generally two-way paved roads that intersect each other frequently, allowing a high degree of access but at slow speeds. Traffic lights and signs help regulate the movement of vehicles along these streets and control the access to some streets. Urban streets are usually classified as one of three types: local, collector, and arterial. These designations are based on the amount of traffic each type is designed to carry.

Most urban streets are residential, or local, streets that allow vehicles to access public and private property. These streets make up 70 percent of total urban-road mileage but handle only 14 percent of all urban traffic. Collector streets convey traffic from residential streets to larger roads called arterials. Arterials are used to get quickly from one point to another and can accommodate high volumes of traffic. In large cities, arterials are often similar to highways in construction even though they are located within city limits. Although arterials account for only 17 percent of the urban-road mileage, they account for most of the traffic in cities.