Definition of Interstate Highway in the United States

Interstate highways, in the United States are generally freeways. It can also be divided into urban freeways and rural freeways. As far as the definition of freeway is concerned, the definition of Highway Capacity Manual (HCM) is slightly different from that of Manual on Uniform Traffic Control Devices (MUTCD) and AASHTO's Geometric Design Guide for Highways and Urban Roads. The MUTCD defines a freeway as a highway facility with at least four lanes in each direction, divided traffic, fully controlled access, and high volumes of high-speed traffic. The MUTCD and AASHTO definitions, on the other hand, do not emphasize at least four lanes in each direction. Freeways in the U.S. have their land reservation placed in the median, so their medians are generally wider. The advantage of this method of forward land reservation is that land acquisition is completed in one go, and later reconstruction and expansion do not affect the developed land on both sides of the highway. Although the initial investment is higher, it can ensure the long-term planning of the transportation corridor. When the expansion is completed, the center strip is usually only the width of a concrete parapet. Of course, freeway medians can also be used to build cross-town rapid transit railroads to promote the development of public transportation.

Traffic control facilities refer to the equipment, facilities, methods and technologies that announce traffic regulations to road users, provide timely warnings and guidance, and promote the orderly, efficient and safe operation of traffic. Traffic control facilities not only regulate the demand of road users for road facilities, but also increase their capacity. Therefore, traffic control facilities are an important means of improving highway efficiency and ensuring operational safety. Without traffic control facilities, it is impossible to have a complete, orderly, efficient and safe highway transportation system.

Traffic signs and markings, as the most basic traffic management facilities, are mainly used to regulate the traffic behavior of users, warn of driving environments that are prone to traffic risks, and at the same time provide users with clear and reliable wayfinding information. Due to the different levels of road facilities, the variety of road users, the diversity of driving behavior, and the driving environment is very varied, so the types of signs, layout design and its matching marking design content is very rich. In order to better serve the road users, signs, markings design is the fundamental standardization, so that road users in the same or similar conditions, the formation of consistent driving expectations, not panic, thereby improving traffic efficiency and safety. And over-emphasis on local characteristics, so that different road users for the same or similar traffic conditions can not form a consistent reflex, not only contrary to the signs, markings design of the fundamental, but also prone to traffic accidents.

By 2006, the U.S. Interstate Highway System had reached its 50th anniversary and has basically reached its design life. However, the main problem facing the U.S. Interstate Highway System is underinvestment, and the original gasoline tax policy is difficult to meet the rising maintenance and expansion needs. Other investment strategies, such as road congestion pricing, private investment in toll roads, or the introduction of other tax policies have been gradually implemented. Another issue is how best to maintain, manage and utilize this existing infrastructure. It has been widely recognized that it is unrealistic to try to solve the congestion problem by building roads alone. So highway authorities, from the Federal Highway Administration and state DOTs down to county and municipal DOTs, have shifted their main focus to maximizing highway capacity, reducing both regular and non-regular traffic congestion, and improving traffic safety. For example, carpooling lanes can significantly increase the occupancy rate of single vehicles (carpooling rate), thereby increasing the transportation potential of highways. Similar measures are already in widespread use.

On interstate highways, major traffic control facilities are concentrated in the interchange areas associated with highway entrances and exits. This includes facilities such as import guidance, exit announcements, lane guidance, speed control, hazardous location warnings, no backtracking, and location distance announcements, and of course, natural connections to local roads and good on-road service. At the same time, the development of carpool lanes and the rise of road toll management are also enriching the content of highway traffic management facilities. For example, a whole set of carpool traffic sign and marking system developed in recent years is a good example. This chapter concentrates on the following types of facilities, i.e., entrances and exits, dedicated carpool lanes, dedicated goods vehicle lanes, rest areas and toll booths.

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