Batch plants, which produce HMA in individual batches, are the older of the two types of HMA production facilities. HMA was originally made in batches; it was not until the 1970s that drum plants became a popular HMA production option. Currently about 70 percent of all operational HMA plants in the U.S. are batch plants while about 95 percent of all newly manufactured plants in the U.S. are drum plants (Roberts, et al., 1996). This means that as older batch plants are retired they are more than likely to be replaced by new drum plants, which can provide greater mobility and production capacity. Typical batch quantities range from 1.5 to 5 tons of HMA. Figure 1 shows the basic components of a batch plant and their functions.