comparative speed limits

As far as I can casually tell, the default urban speed limit in the US, insofar as we have such a thing, is 30 MPH (50 kph). Major/arterial streets might be 35-50 MPH (56-80 kph). Georgia state law caps allowable residential speed limit as high as 45 MPH (72 kph). School zones might be 25 MPH (40 kph), maybe lower.

Some cities such as Boston and Seattle have been lowering that a lot. Seattle seems to be moving to 25 MPH (40 kph) everywhere. Boston has lowered the default from 30 to 25 MPH (50 to 40 kph), likewise Somerville MA. Cambridge MA has reduced from 30 MPH to 20 MPH (32 kph) on local streets and 25 (40) on arterials.

OTOH Japan is 30 kph (18 MPH) for residential (often one-lane, one-way, no sidewalk, there's also an option for 20 kph (12 MPH)), 40 (25) for urban 2-lane roads, 50 (30) for 4-lane roads, or no more than 60 (36 MPH) on divided 4-lane roads with low pedestrian activity. Nothing with at-grade intersections, or with pedestrians and cyclist access, can be faster than 60 KPH.

So Japan residential streets are slower than typical US school zones. Most major streets are no faster than typical US residential streets.

Wiki says the Dutch urban default is 50 kph, but 30 kph residential is common, and "home zones" may be as low as 15 kph (9 MPH), but some 4-lane arterials are posted at 70 kph (44 MPH)

Makes a big difference in crash safety, but also in car noise: slower cars are quieter, and probably fewer in number.

Canada defaults to 50 KPH, but Montreal lowered its streets to 30 residential, 40 arterial.

Australia seems similar to the US, though with pushes to 40 kph in central business areas.

This road in Osaka, an six lane (or here, eight) divided road with barriers to discourage pedestrians (and prevent them from jaywalking)... is 50 KPH (30 MPH). It comes out of giant intersection, which has a 40 kph (25 MPH) sign posted.

random observations

North Oakland has become rather aggressive about traffic calming on residential streets. Lots of circular planters in intersections, like a micro-rotary, to slow cars down; that's been around for a while. But new was lots of barriers: say you're allowed to enter a street, then you probably passed a barrier blocking exit in the opposite direction. And when you reach the end of the block, you'll be facing a barrier across the intersection, so that you'll have to turn left or right rather than go straight. (Legally, anyway -- I saw a few drivers cheat.) So the streets are internally two-way, but a maze of limited accesses to prevent traffic from running through.

Lots of bicyclists without bike helmets in Montreal. I'd guess helmets are like 50%, though I haven't really counted. Common, unlike Osaka or Amsterdam, but not as ubiquitous as I recall from the US. Some of that is people on Bixi bikeshare, but I've seen road bikers in traffic without a helmet. There did seem to be more helmets yesterday on the canal shared path, but that's the sort of thing that probably attracts faster biking (apart from having to share with the pedestrians.)

More bikers than I was used to in LA, not sure about Boston or Oakland. I see a lot on Maisonneuve but it has a physically protected bike path.

Montreal forbids biking on the sidewalk, unlike LA. The one sidewalker I recall seeing was laboring uphill toward Mont Royal. Can't say as I'd be happy to bike on the major streets here.