"Halfway to Hell" marks the middle of the bridge.
The 182.2-smoot mark is accompanied by the words "Halfway to Hell" and an arrow pointing towards MIT. Each class also paints a special mark for their graduating year.
The "smoot" is named after Oliver R. Smoot (class of 1962), an MIT fraternity pledge to Lambda Chi Alpha, who in October 1958 was used by his fraternity brothers to measure the length of the Harvard Bridge between Boston and Cambridge, Massachusetts.
One smoot is equal to his height (five feet and seven inches ~1.70 m), and the bridge's length was measured to be 364.4 smoots (620.1 m) plus or minus one ear, with the "plus or minus" intended to express uncertainty of measurement.
I noticed the Golden Gate Bridge has a HALFWAY-TO-HELL CLUB. The most conspicuous precaution was the safety net, suspended under the floor of the Bridge from end to end. During construction, the net saved the lives of 19 men who became known as the "Half-Way-to-Hell Club."