Major Types of U.S. Roads

Traffic control facilities such as traffic signs and markings are closely related to the conditions of the road itself.

Due to the different levels of roads, their geometric curves (such as flat curves, longitudinal curves, etc.), sight distance and pavement type and other design standards are different, which will directly affect the location of traffic signs, size and its role in the effect and so on.

The standard width of the lane of the U.S. road is 3.6m (12ft), the road design includes site width, speed, traffic volume, sight distance, road width, number of lanes, flat longitudinal line, cross-section, slopes, bridges and culverts, drainage, intersections and traffic control.

Depending on the conditions and requirements, roadway design also considers levels of service, guardrails, on-street parking, landscaping, lighting, bicyclists and pedestrians.

Understanding the classification and function of U.S. roads will help the reader better understand the installation and requirements of their traffic control facilities. Depending on the specific classification purpose, different methods can be used to classify U.S. roads.

For example, according to the geometric design standard of the route, it can be divided into highway and ordinary highway types; according to the highway number, it can be divided into Interstates, U.S. Highway, State Road and County Road.

For transportation planning and economic development, roads are classified by their function.

By roadway function, we mean the services provided by the roadway and their characteristics, etc.

A fully functional roadway system takes into account the six basic constituent phases of a traffic trip, including the arterial travel phase, the transition phase (on-ramp travel), the secondary roadway travel phase, the feeder travel phase, the approach phase, and the arrival phase.

Typically, arterial travel is a high-speed continuous flow. When approaching a destination, there is a transition to moderate speed secondary arterials, then to feeder roads, and finally to local roads to reach the destination.

Roads with different functions serve different travel stages, e.g., freeways primarily serve the arterial travel stage.

When categorized according to function, roads are classified into four categories: arterials, secondary roads, feeder roads, and local roads according to the two main factors of Land Access and Mobility.

The functions of roads in rural areas and cities differ due to differences in land use, road network density and travel patterns between urban and rural areas (suburbs).

An urban area is defined as an administrative district with a population of at least 5,000, while all others are rural areas. Based on the functional classification of roads, the proportion of roads with different functions, and the amount of traffic they carry, as provided in the Geometric Design Guide for Highways and Urban Roads published by AASHTO, it can be summarized that local roads make up the majority of the road network in terms of length, both in the rural areas and in the urban areas.

However, the proportion of total traffic carried by local roads is very small, e.g. not more than 20% in rural areas and generally not more than 30% in urban areas.

Specifically, local roads primarily provide access to residential areas, commercial areas, farms, and some may carry limited transit traffic.

Most local roads in the U.S. are designed for no more than 2,000 vehicles per day (vpd), and when the design volume falls below 400 vpd, the roadway may become unreasonably cost-effective.

Most local roads are two-lane, and for single-lane local roads, avoidance lanes are provided at regular intervals.

The design speed of local roads is related to the design traffic volume and its topography, which generally varies between 30-80km/h.

For local roads in urban areas (city streets), the road cross-section is different from the rural local highway cross-section.

In addition to lanes, the urban road cross-section also takes into account parking strips, medians, curbs and drainage facilities.

In addition, the end of the road, alleys and walking paths, etc., to consider such as access, turnaround and other use requirements.

For local roads in tourist, mining and forest areas, there are also different requirements for their design and marking.

According to the specific circumstances, local roads should be set up with necessary traffic signs, markings and signals and other traffic control facilities.

Spur roads are roads connecting local roads and main roads, which not only bear the traffic between local roads and main roads, but also bear the local transit traffic.

Feeder roads are generally one-way two-lane roads with design speeds varying between 30 and 100km/h.

Feeder roads bear medium traffic, design traffic volume in rural areas should be considered as 20 years, in urban areas can be considered as 10-20 years.

Traffic control facilities of feeder roads should be unified and consistent.

Arterials are high speed, high volume roads that connect major urban or rural locations.

In rural areas, arterials include all interstate highways and freeways, multilane highways, and some two-lane highways between cities; while secondary arterials are primarily some of the roads that connect urban centers to surrounding small towns.

Urban arterials include freeways and some two-lane urban roads with high traffic volumes.

The daily traffic volume of highways in rural areas is mostly tens of thousands of vehicles, and the traffic volume of urban highways is generally not less than 200,000 vehicles per day .

The design speed of arterial roads generally varies between 50-130 km/h.

Compared to other types of roads, the design of arterials takes into account the level of service, rest areas, climbing lanes, entrances and exits on a case-by-case basis.

Effective access control management methods help to ensure that the level of service of the arterial road, commonly used control methods are legislative control, zoning control, lane control, geometric line control (such as the establishment of auxiliary roads, intersections) and so on.

It should be especially emphasized that the entrances and exits of freeways must be completely controlled, and their ramps should be specially designed to ensure traffic safety and unimpeded flow.

When going through a city, the highway may be in the form of a roadway or viaduct.

In Boston, a famous project used to bury the downtown highway deep into the form of a tunnel, avoiding the isolation of the highway from the community and the destruction of the landscape.