Individual motor vehicles as a transportation mode can only provide low-density service, even in the best of circumstances certainly as compared to most public transit operations. Automobiles are large physical units requiring considerable space, as discussed earlier, and they require buffer zones because they are operated by drivers with different skill levels.

When limited-access highways were first built at a large scale, it was determined that the base capacity of a lane (with no stops or obstructions along the way) was 1200 to 1500 vehicles per hour. ^[69] Over the years this official figure has moved upward, and today it stands at 2000 to 2200. ^[70] Theoretically, a lane should not carry more than 2200 to 2400 vehicles per hour under safe conditions, ^[71] since such volumes would be difficult to maintain in a steady state over an entire hour. Furthermore, there are always physical or operational impediments that reduce the base capacity to what is called practical safe capacity. This includes a series of conditions that make drivers react instinctively to constraints and reduce driving speeds:

• Lanes narrower than 12 ft

• Lateral clearance of less than 6 ft (vertical elements of any kind)

• Frequent access points

• Steep grades above 3 or 5 percent

• Sharp horizontal curves

• More than 5 percent heavy vehicles in urban areas

There are tables and equations that allow in each given case a precise calculation of the real capacity by applying reduction factors.