Road transport is the most ubiquitous means by which humanity worldwide shuttles from one place to another. For example, in the United States, the more than 6.58 million kilometers road network enables users to travel and move goods across the vast country with ease. However, it could come as a surprise to know that the road network is a composition of different types of roads, which is the subject of this piece.
Roads come in various classifications, depending on accessibility, usage, type, and heaviness of traffic, among other factors. Because of the extensiveness of the elements, the number of the kinds of roads is also as vast.
A short trip in most countries anywhere in the world will open your eyes to long highways, boulevards, backstreet alleys, and small unpaved paths leading into the wilderness. More often than not, the type of road defines its usage. Although road classifications might appear trivial, it helps to understand the process behind them.
Roads In The Past
Roads have undergone a tremendous transition over millennia as societies and their priorities evolved. For example, more than 10,000 years ago, most roads were simple unpaved paths because they only supported feet, hooves, and wooden wheels.
However, dusty paths in hot weather suddenly became an unpassable mud track in the wet season. Thus, it became an urgent priority for people in Mesopotamia to make the roads more transit-friendly, especially within cities.
Archaeological evidence suggests Mesopotamians paved roads with thoroughly dried clay bricks around 3000 B.C. They used bitumen to hold the tiled bricks together, hence making them partially all-weather. But because of the labor-intensive nature of paving the roads, they took on a particular societal function. They were only available for special use, such as military or religious activities.
At its height, the Roman Empire pioneered some groundbreaking road technology, long after the Mesopotamians. The Empire constructed more than 50,000 kilometers of highways through Asia Minor and Europe, primarily for trade and military use.
The roads were paved with hard stones after laborers made recesses into gravel and rock layers into the ground. Then, the rocks were thoroughly polished to make durable tops. Interestingly, some of the roads paved in this region, in around 100 A.D., survived till the time of writing, such as Via Appia in Rome.
About four centuries later, the Roman Empire collapsed, but road technology continued to evolve. For example, the Industrial Revolution in 1700s Europe saw the advent of long gravel highways connecting local governments. In England, for instance, an extensive road network emerged to connect cities. Other roadways like turnpikes sprang up in the rural areas connecting farms with major cities like London.
However, the roads were often impassable in wet weather because of inferior technology. They were paved with small stones placed on mud and without any form of drainage. Later, John Metcalf introduced a novel road design, where each side of the surface was slopped to drain excess moisture.
Nonetheless, it was not until the 19th century that the world saw the first durable roads. At this time, a Scottish engineer called John McAdams pioneered the approach where tiny bits of crushed rock would be sprinkled on the Metcalf roads and pressed firm.
In the same period, engineers in the United States started binding the rock bits with bitumen, making them more durable and passable in all weather seasons. This approach opened a new era of asphalt roads, which rule to the present.
Roads In The Present
The modern road is an engineering masterpiece. However, this transformation was long and detailed.
Road technology took the backseat in the mid to late 1800s as railroads reigned supreme. It was not until the invention of the automobile that road improvements took on a new significance.
However, most roads built from the turn of the 20th century onwards were merely an improvement on the Macadam technology pioneered a century prior. For example, engineers in apartheid South Africa laid a solid foundation of rock and cement and smoothed the top without using bitumen. Elsewhere, like U.S. and Europe, engineers refined the bitumen black-topped roads to minimize friction and rolling resistance.
While asphalt continues to construct most of our roads, engineers worldwide are researching alternatives. For instance, Germany built highways with limestone bases, which are famous for their durability. The roads are thicker than the traditional asphalt-made, although they are pricier.
Other emerging alternatives to asphalt in present-day road technology include rubber, plastic, and food scraps. The rubberized top is gaining currency primarily because it is quieter than asphalt. Also, the bouncy nature of rubber makes the roads safer.
On the other hand, India is experimenting with melted plastic to construct more durable roads. The government seeks to trial the novel technology in byways and more minor roads before taking it to major highways and freeways.
Road Classification System
What Is Its Purpose?
The current road classification system is as old as the history of road technology. The system divvies up a road network into specific road types, each performing a particular service in facilitating transport between two points.
Since the age of the automobile, roads have been classified based on how they facilitate vehicular transport. Other than generating an orderly grouping of roads, the system provides a reliable tool for traffic management in cities and countrywide infrastructure development. This includes road infrastructure design, better transportation planning, and infrastructure maintenance.
Moreover, the road classification system helps control air and noise pollution in specific parts of the city/countryside. Finally, it defines the traffic capacity of each road, where lower traffic volume roads suffer minor breakdowns than higher traffic volume roads.
Also, the system helps manage the safety of road users and property. For example, the system defines specific traveling speed limits for each road category, guaranteeing safety.
Lastly, a road classification system enables authorities to measure the total road network in their jurisdiction. Usually, one can find official records spelling out the complete road network but under different classes. This classification also facilitates an efficient transport system across the country.
Furthermore, correct classification of roads will direct drivers to proper directions to avoid excessive destruction. For instance, truckers can use the information to avoid poorly paved roads that might lead to the vehicles getting stuck and causing a massive traffic jam.
Factors Influencing the Road Classification System
A road could be a thoroughfare, a freeway, or a minor pavement in a homestead. But, how can one tell the difference, especially when one is less informed about civil engineering? Over the course of the history of road technology, engineers came up with various factors along which they divide roads into specific classes. They include:
Accessibility of the road and permitted speed
Different roads often allow vehicles to cruise at different speeds and have controlled access. In addition, the roads on which cars can drive at the highest rate are continually developed to ease traffic congestions. The roads are devoid of speed breakers, road signs, footbridges, rails, and parking spaces.
Because they allow fast-moving traffic, such roads are out of bounds for users like cyclists and pedestrians. Some of the classes of roads under this category include freeways, highways, arterials, local roads, and collectors (we will dive deeper into each class later).
Construction material used to build the road
If anything, the sneak peeks into the history of road technology tell you that different epochs are defined by the building material used. In the primordial era, for example, roads were mounds of soil pressed firm. Later, as we saw, the Mesopotamians improved the roads by tiling hardened bricks and binding them tightly with bitumen.
Further improvements introduced asphalt roads, which consist mainly of the road networks worldwide today. Because many roads today are built with diverse materials, the classes are also various. They include earthen, murram, gravel, cement concrete, bituminous, and Water Bound Macadam (WBM) roads.
Who controls the road? It could be the national government or local authority. In this case, the roads are classified according to the authority exercising legal control over the road. This means you’ll have classes like national or state highways and district or village roads.
Surface type or rigidity
Roads also come in different levels of rigidity, which is a function of the building material used. For example, asphalt and concrete roads fall on the rigid end while pavements are made of simple materials like soil and gravel area on the flexible end.
The road’s rigidity affects the load it can support, hence the type of vehicles allowed. On the one hand, tough roads support heavy usage, therefore popular in high traffic regions like cities. On the other hand, flexible roads like pavements are familiar in areas popular with pedestrians and cyclists.
The earth’s topography is diverse, ranging from plains to hilly areas. Yet, interestingly, humanity has had to conquer the environment in the quest for expanded connectivity via the road network.
For this reason, you’ll find roads in all manner of topography. If a route is located in a plain area, with fewer bends and hills and valleys, the road is categorized under the plain road class. On the contrary, you’ll have a hilly road if the topography is steep and mountainous.
The location also matters because it affects the traffic volume flowing in that particular area. Some classification systems fold together roads influenced by location and legal control into one class, such that you’ll have national or village roads. However, you could also imagine roads in this category as rural or urban roads.
Roads also serve various functions. As you may recall, freeways and highways serve areas with high traffic and only allow vehicles to drive at high speeds. But between the freeways/highways and village roads lie several other types, each serving a specific end.
A good example is a collector road. As the term suggests, such road types collect traffic from arterial and local roads and discharge them onto highways/freeways and vice versa.
Unlike local roads, collector roads have a higher speed limit, and they exclude facilities like parking and footbridges, though not invariably.
Classes of Roads
From the preceding, there are numerous types of roads. However, some countries have developed classification techniques that define roads into five major classes. They are:
A trunk road is sometimes referred to as the primary road network with national significance. In the United Kingdom, trunk highways fall under the purview of the Highway Agency. Across the Atlantic, in the United States, the trunk highway system includes the interstate road network and some state highways.
Roads in this class often have specific unique numbers for ease of identification on GPS systems for drivers. For example, a road user in the U.S. State of Minnesota might see their GPS read “Trunk Highway 62” (some GPS devices might call it “State Highway 62”) when driving through Inver Grove Heights from Minnetonka. However, the driver might know the road’s name as being Crosstown Hwy. 62.
Regardless of the appearance of the highway’s name on your GPS device, you might notice that the number at the end does not change. This is because the number is the road’s official designation by the country’s road classification authority. So, to avoid confusion, always remember the road’s number when reading your GPS device or map.
As the name suggests, a trunk road is a primary route connecting a region’s most significant installations, including cities, airports, ports, and more. Like a tree’s trunk, it allows traffic to flow from one end of the country to the other.
Some of the roads that fall under this class include:
- Freeway – it is a highway exclusively designed for high-speed vehicular transport. According to the Arizona Department of Transport (AZDOT), a freeway is not your typical highway because it entails controlled access. Usually, traffic flow on such a road is unhindered because there are no intersections, traffic signals, or pedestrian crossings. Moreover, vehicles traveling in opposite directions are separated such that one cannot cross over unless with the help of an overpass/underpass. Nevertheless, freeways are a common feature in urban regions.
- Highway – according to AZDOT, all freeways are highways, but not all highways are freeways. The primary difference is that freeways are controlled-access, made possible through slip roads (or ramps). On the contrary, highways have traffic signals, cross-traffic (or intersections), and, in some cases, pedestrian crossings. Additionally, highways have lower speed limits than freeways, typical in peri-urban and rural areas.
- Arterials – these are crucial trunk roads, especially in cities that experience substantial traffic volumes. Also called urban roads, arterials are the primary road network inside big cities. The roads carry traffic from within cities and pour it onto freeways for further flow into other cities or onto highways and into rural areas. Here, speed limits are lower, and the roads have plenty of footpaths, cross-traffic zones, and, hence, lots of traffic signals.
Given the nature of trunk roads – as primary roads linking different parts of a region – they are mainly high-tech pieces of work. The roads are usually bituminous with a hard surface to handle substantial traffic volumes without wearing off quickly. Another material used for such roads is concrete.
This class of roads encompasses the transportation system that distributes traffic within a region. Characteristics of such roads include high traffic volume, high-speed limits, and (in some cases) restricted access. As such, one might think of this class as a superset of all the vast highways, including freeways, major arterials, and highways.
In a rural setting, primary distributors could be highways and arterials but without restricted access. The issue here is that rural areas often experience low traffic volumes, which excludes the need for high speeds. Therefore, such roads often include pedestrian paths, parking locations, and other at-the-roadside amenities that you’d rarely find in urban areas.
Additionally, primary distributors in rural settings are not necessarily hard-surface roads. Instead, the roads are usually earthen, murram, WBM, or gravel in nature.
A great example of primary distributors is what engineers call collector roads. A collector road collects traffic from local roads (either local urban or rural) and spills it onto arterial roads. Usually, vehicles travel at lower speed limits on collector roads, but they can push the needle a notch higher when they hit the arterial road network.
However, the distributor class of roads is broad because you’ll often meet subclasses like Distributor A, B, C, and so on. Therefore, for ease of discussion, we will adopt another classification in the hierarchy of roads where district or local distributors follow primary distributors in the pecking order.
District/local distributors sometimes go by the name Transition Roads. This is because these roads are often without direct access to residential areas and other properties. Instead, they exist within more significant developments where they transition road users from high traffic volume roads (Primary Distributors) to residential access roads.
This class of roads is replete with junctions and intersections, and speed limits are significantly low. Nevertheless, it helps to note that the motorist enjoys priority on such roads.
Another characteristic of local distributors is that they are two-way. These roads also have pedestrian crossings, and the speed limit is often throttled down to about 20 mph (about 32 kph).
Also, this class of road is the route where local bus services operate. So the roads will have bus stations and many other facilities that come with the pedestrian shuttle services. You’ll also find railings on the roadside to keep pedestrians safe.
Residential Access Roads
Unlike the local distributors, residential access roads take users into private residences and other property. Moreover, in their layout, these roads prioritize pedestrians because inhabitants mainly use them for walking, playing, and jogging.
Because of the low traffic volume typical on such roads, the paving is often light. As a result, the quality of material used in working on them is often low cost, and maintenance works are frequent.
Also, the speed limit here is at the slowest end because most roads have parking facilities at the side. Most residential access roads are one-way, and the lane size is small (often 5.5 meters, excluding on-street parking and other facilities.
Minor Access Roads
These are the small loops of roads that serve a handful of dwellings, although some are cul-de-sac. The carriageway for such roads is thinner than all the other roads, often 5.5 meters or less. Also, the speed limit here is at the lowest end, typically sub-20 mph. Because of high pedestrian usage, the road comes with pavements for various uses, including jogging.
Minor access roads are familiar in residential estates, a common feature in the suburban areas dominated by dwellings. The streets are usually paved with gravel, although it is not uncommon to find some of them being murram or earthen.
Other Classifications of Roads
Given the factors discussed earlier, it is apparent that road classes are extensive. However, we have covered most of the primary types in the sections above. But extensive research into this topic identifies specific classes that might seem trivial at first but are an essential part of the global road classification system.
For example, you’ll hardly miss roads classified based on weather. Under this class, you’ll find all-weather and fair-weather roads.
An all-weather road is a surface on which users can traffic in all conditions of weather. It means the road does not collect mud that makes it impassable when it rains, nor does it become too slippery.
Typically, an all-weather road has a rigid surface, often made of asphalt, bitumen, or concrete. Such a surface repels water onto the side of the road. Also, it means that this type of road includes an efficient drainage system that carries runoff water away to avoid seepage into the soil on the underside.
Examples of all-weather roads include freeways, highways, arterials, collectors, and local distributors.
Over the years since McAdam, engineers have been working on the technology to keep roads usable in all weather conditions and to be able to survive long-term usage with minimal repair requirements. To this end, the experts discovered that the following conditions are necessary for all-weather roads:
- Good location – good roads must be located in areas where the soil is unlikely to be waterlogged after heavy rainfall. Therefore, engineers must select areas with suitable soil structures.
- Good crowning – an all-weather road must keep surface water moving, meaning enough care should be taken to crown the surface to an appropriate slope. Also, one must ensure that the road’s crown is uninterrupted from the center to the side.
- Intercept subsurface seepage – seepage is common in sidehill areas and places with heavy annual rainfall. In such a case, it is critical to install sub-drains to intercept the water before it seeps into the roadway.
- Select a suitable gravel – gravel makes up the upper part of the road’s foundation. After choosing a good location and laying the bottom foundation to the right measure, the next most crucial part is the gravel (in a gravel road). First, engineers must examine the gravel sources to determine the gravel’s suitability. This entails estimating the percentage weight of gravel, sand, clay, and silt to assess quality standards.
Such roads are only passable in fair weather. It means you’ll have to find an alternative route if it rains heavily. Usually, these roads have a flexible surface, meaning they are paved with murram or pressed earth (earthen roads).
The type of roads in different regions often depends on the budget allocated to construct them. As such, low-income areas often have fair-weather roads whose conditions worsen in the rainy season.
In developed regions, road technology is moving at a fast pace. For instance, engineers in the Netherlands are working on a freeway concept that repairs itself. The scientists discovered that lacing asphalt with steel-wool fibers turns the road surface “into one big conductor.” When this road cracks, one can pass a massive magnet on a truck, make the asphalt contract, and then close the gaps.
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Star Tribune: Minnesta trunk or state highway, what’s the difference?
Local Government & Municipal Knowledge Base: All Weather Road