For street photography people I would say 40% use a 50mm lens, 40% use a 35mm while the rest are either shooting very wide or with a mild telephoto.
They call the 50mm the standard lens for full frame 35mm photography but actually its more like 43mm but I think Pentax are the only manufacturer to actually produce one, but in the past 40mm and 45mm lens were very popular with street photographers, and with the explosion of interest again in fast 50mm lens rather then zooms, small 40mm lens are now also making a comeback.
Canon had one for their EOS mount which in its pancake format made for a lovely little everyday walk around lens, far easier to carry then a bit heavy zoom.
With mirrorless cameras offering the chance of small bodies and small lens it seems strange that no-one has released a small 40mm. Instead we have large very fast heavy 50mm f/1.2 and 85mm f/1.2, but this week Nikon announced their new 40mm f/2. A small light lens with a plastic mount. Its nothing like their un-compromised and optically stunning Z 50mm f/1.2 but then its not over £2000 and weighing over a kilogram, instead its £250 and weighs 170g so is the ultimate always carry with you lens. Lets hope more manufactures jump on this.