Final answer:
Taxicabs must adhere to posted speed limits on U.S. or state highways, which during the 1970s were typically set at about 55 mi/h. Modern limits vary, and minor overages are often tolerated, but significant breaches can lead to tickets.
Step-by-step explanation:
The maximum speed for a taxicab on a U.S. or state highway varies by jurisdiction and the specific limits posted on road signs. However, during the 1970s oil crisis, maximum speeds on U.S. highways were set at about 90 km/h, which is approximately 55 mi/h. This limit was established to help conserve fuel, acknowledging that at highway speeds, over 50% of the power of a car is used to overcome air drag, and the most fuel-efficient cruising speeds were determined to be around 70-80 km/h.
Nonetheless, current speed limits might be higher in some areas, and taxicabs, like all other vehicles, are required to adhere to these posted speed limits. A speed limit sign, such as one indicating 30 miles per hour as in Jacinto City, Texas, should be followed, and while a minor speed overage is often tolerated due to speedometer inaccuracies, significant breaches can result in traffic tickets.