What is a Mansfield bar; and, why is it so named?

A bumper bar to prevent motorists from driving under large vehicles a la Jayne Mansfield.

http://sparebumper.com/index.php?act=viewProd&productId=48

The 'Mansfield Bar' is an underride guard which is an assembly hanging down from the bottom of the rear of a semi-trailer. It is intended to provide some protection for cars which start to run into the rear of the trailer. The bottom of the rear of the trailer is near head level for an adult in a car, and without the underride guard, the only protection for an adult's head in such an accident would be the car's windshield. This came into use in the aftermath of the accident that killed Jayne Mansfield on June 29, 1967.

https://www.autoweek.com/car-life/but-wait-theres-more/a2142281/heres-why-those-extensions-semi-trailers-are-called-mansfield-bars/

On June 29, 1967, Mansfield sat in the front seat of a car that slammed into the rear of a big rig. The car slid under the truck, instantly killing all sitting in the front seats, including Mansfield.

Soon after, the National Highway Traffic Safety Administration proposed new standards for underride prevention guards, a requirement that was not fully implemented until 1998, according to the NHTSA. Back then, the government agency estimated fatalities from underride accidents each year in the U.S. at between 200 and 300.