Pavement Facts

There are over 3.96 million public centerline road miles (8.28 million lane miles¹) in the U.S. and of this, 2.52 million miles (or about 64 percent) are paved (FHWA, 2002a). This section contains several quick pavement facts that serve as a good basic background for the rest of the Guide.

Pavement Purpose

Typically, pavements are built for three main purposes:

• Load support. Pavement material is generally stiffer than the material upon which it is placed, thus it assists the in situ material in resisting loads without excessive deformation or cracking.
• Smoothness. Pavement material can be placed and maintained much smoother than in situ material. This helps improve ride comfort and reduce vehicle operating costs.
• Drainage. Pavement material and geometric design can effect quick and efficient drainage thus eliminating moisture problems such as mud and ponding (puddles).

Pavement Categories

Pavements can be broken down into two broad categories:

• Flexible pavements. These are asphalt pavements (sometimes called bituminous pavements), which may or may not incorporate underlying layers of stabilized or unstabilized granular materials on a prepared subgrade. These types of pavements are called "flexible" since the total pavement structure bends (or flexes) to accommodate traffic loads. Flexible pavements comprise about 93 percent of U.S. paved roads (about 98 percent of Hawai'i roads).
• Rigid pavements. These are portland cement concrete pavements, which may or may not incorporate underlying layers of stabilized or unstabilized granular materials. Since PCC is quite stiff, rigid pavements do not flex appreciably to accommodate traffic loads. Rigid pavements comprise 7 percent of U.S. paved roads (about 2 percent of Hawai'i roads).

Hot Mix Asphalt (HMA) Defined

Hot mix asphalt (HMA) is a bituminous concrete made principally from asphalt binder and aggregate. It is distinguished from other bituminous products by its constituent materials (asphalt and aggregate), mixture design methods and elevated mixing temperature (thus the term "hot mix"). Although it is known by many different names such as hot mix, asphalt concrete (AC or ACP), asphalt, blacktop or bitumen, this Guide makes a conscious effort to consistently refer to this material as HMA. Other types of asphalt-based pavement surfaces discussed in this Guide such as fog seals, slurry seals and BSTs are not HMA but are nonetheless important to the pavement industry. [More information]

Tar vs. Asphalt

Tar and asphalt are two different materials. Tar, often called "coal tar" is a byproduct of the destructive distillation of coal to form coke. Coal is of plant origin and was formed in swamps similar to present-day peat bogs and in lagoons, probably partly from plants growing in the area and partly from plant material carried in by water and wind (The Columbia Electronic Encyclopedia, 1999). Asphalt is a petroleum residue left over from the distillation of crude oil. Crude oil is the result of incompletely decayed ancient plant and animal remains. The carbon chains within tar are, in general, slightly shorter than those in asphalt. In pavement applications, tar is generally more brittle and less elastic than asphalt causing it to crack under typical loading and environmental conditions. Tar will, however, not dissolve as easily in the presence of lighter petroleum distillation products such as gasoline and kerosene. While asphalt is used throughout the pavement industry, tar is generally limited to sealant use.

Road Use

U.S. and State roads are being used at an ever increasing rate. Although U.S. road centerline miles have only increased by about 10 percent since 1960, U.S. registered vehicles have increased by over 300 percent and vehicle miles traveled have increased by more than 380 percent over that same time. In sum, our road network, which has not significantly expanded since 1960, is now carrying over 3.8 times the number of vehicles. Moreover, truck (the most damaging type of vehicle) vehicle miles traveled (VMT) is increasing at an even faster rate than automobile VMT. A typical tractor-semi trailer combination averages 100 - 200 miles/day in the U.S. for a total of 35,000 - 70,000 miles/year, which is substantially more than the typical passenger vehicle (USDOT, 2000). Thus, pavement loading is growing at an even faster rate than traffic.

HAPI Pavement Note on Hawai'i Road Use

Hawai'i has 4,278 centerline miles of roadway (9,258 lane-miles¹). As of 2002, Hawai'i's population was just over 1.2 million and of that about 950,000 (or 78%) were of driver age with about 788,000 licensed drivers and over 868,000 registered vehicles. These vehicles traveled approximately 8.7 billion miles in 2001 at an average of just over 10,000 miles per vehicle (7,109 miles per capita).

Figure 1: H-1 Freeway near Nu'uanu Avenue

-data from the FHWA's Highway Statistics 2001. http://www.fhwa.dot.gov/ohim/hs01

Figure 2: Growth of Vehicle Miles Traveled, Registered Vehicles and Statute Miles of Roadway in the U.S. Since 1960 (from FHWA, 2002b)

Other Pavement Uses

HMA is used in many applications beyond roads. HMA is by far the most popular airfield pavement and can also be used in drainage swales, retention pond and reservoir applications as well as other specialty applications (see Figure 3).