This same pre-warning scheme is installed on Eisenhower(?) Parkway before the M&E crosses it at grade. There is a stiff hill and a short descent just before the tracks; going ~50MPH and having to slam on ones breaks in about 100 feet of roadway would make for many accidents. The crews cross rt. 10 & 46 with only lights - the conductor sometimes wont even get on the ground at these locations, just hang off the front of the engine with a flag and wave the engineer foward -much safer IMHO-
There's a warning sign like that west of the intersection with New Providence Rd. in Mountainside, so I suppose that would help.
As to the "speeding motorists," I'm sure those like my mother are more worried about times when it is impossible to speed. Just look at Rt. 22 westbound in Union (between the footbridge and the ex-Harrows lot) from about 10am to about 7pm any day of the week. It's bad enough already. Then again, the railroad would serve as both another break in the traffic, and as another inducement for through-traffic to take I-78.