re: Why is the I-10 bridge over Pearl River so high?

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The bridge was built to the standards of the time it was built. At that time people wouldn’t travel 70 on the interstate.

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Bridge was completed in 1970. Speed limit on it at the time it opened was 70 mph, which means (according to the roadway civil engineers in my office) it was built to a design speed of 80 mph.

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You might be lucky if the speed limit was 55 and your car would go only go faster than 55 or 65.

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Maximum speed limits were reduced to 55 in 1974 as a result of the National Maximum Speed Law (NMSL), which was a provision of the federal government's Emergency Highway Energy Conservation Act which was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis. In 1987 speeds were able to be raised back to 65 mph; in 1995 Congress lifted all federal speed limit controls in the National Highway System Designation Act returning all speed limit determination authority to the states.

And unless you had a real POS, all cars back then could easily go 75-85 mph.

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I can remember my family station wagon in the mid 80s with the 55 circled and the speedometer stoping at 85.

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That speedometer situation only lasted three years.

In 1979 the National Highway Traffic Safety Administration (NHTSA) began REQUIRING speedometers to have special emphasis on the number 55 and a maximum SHOWN speed of 85 mph. Some manufacturers circumvented the rule by including extra lines beyond 85 to show higher speeds (again, because most cars COULD go faster than the 85 mph indicated as the top speed on a speedometer).

However, in 1981, the NHTSA eliminated speedomoter rules because they were "unlikely to yield significant safety benefits" and "highlighted '55' on a speedometer scale adds little to the information provided to the driver by a roadside speed limit sign."