I did used to do the descent of Juaquim Miller Road from Skyline Blvd over Oakland, CA on a 42-17 with brakes but not using them. I was in my 20s, had raced two years before and had around 15,000 fix gear miles on my legs. I was also doing work that I knew had long term health issues I would eventually see. My post head-injury crazy years. To keep my sanity, I would ride up Juaquim Miller, turned around at Skyline and ride down. Fully crazy. Juaquim Miller is a parkway, two lanes either side of a planted meridian. Speed limit 50. I never got passed. (I doubt I hit 50 but I was close.)
42-17 has the nice feature that at 20 mph, one's cadence is 100 rpm. Easy math. 50 mph is 250 rpm. Been very close. But as a very experienced fix gear rider with recent race legs. Young and crazy. On a very funky Peugeot UO-8 that had been crashed many times and was as straight as a soggy noodle! (But newish Binda toestraps pulled tight! I wasn't all nuts.)
Ben