Here [1] is an electric-assist bike I recently purchased which can be configured to:

* Measure one's pedal torque and cadence, thus calculate and display "Human Watts" input to the bike while cycling (update rate = once per crank rev)

* Supply a configurable amount of electric-motor power assistance directly proportional to one's "Human Watts" input -- at selectable, configurable, default power amplification factors of 0x ( assist off ), 1x or 2x.

With this instrumented, pedal-power-amplified bike I've found:

* I won't break a sweat if I keep my "Human Watts" below about 30 to 60 (power assist on or off, doesn't matter)

* Breaking 100 - 150 "Human Watts" for more than a few seconds will break a sweat for me.

* (Off topic ) Efficiency comes out to an excellent 2 to 4 Watt-Hours-electric per mile with my settings and low-power riding style, having gotten over 100 miles on my first, not-yet-fully-depleted battery charge.

It's also possible to configure a throttle power control to deal with hills where my "no sweat" rate of 50 Watts -- even with 2x power assist -- won't make it up a grade.

But I configure the throttle as disabled as I don't have hills like that on my commute. I config pedal power amplification for a cut-in speed of 6 mph and a cutoff speed of 13 mph just right so as to stay dry and safe for me and those around me. Un-assisted cyclists frequently pass me; that's fine.

I stick to what roads on my commute have painted bike lanes. My scariest road encounter (a motorist who crowded me toward the curb with his car, twice) would have been impossible were I riding in a protected bike lane and making the kind of protected right turn this new design affords.

Edits for clarity.

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[1] http://www.ebikes.ca/shop/edgerunner/full-bikes/edgerun-stok...

Disclosure here - I'm a racer, own some 10 odd bikes for various disciplines, but realize my 4 mile commute to the loop every day does nothing to further that. It's essentially the "garbage miles" your typical racer talks about. As a result, I typically ride at a pace into work that is focused on not sweating as much as possible, but there are 4 months out of the year that it's so cold out that I've got to put on enough clothes that unless I get it JUST right (almost impossible in chicago) I have to err on the side of being too hot, and 2-3 other months where no matter if I'm wearing my fancy near-mesh high tech Assos jersey or not, barely going at a pace to beat walking, I'm going to be soaking wet from the heat.

As far as "Separated" I guess it becomes important to define separated. If there were physical curbs between, I could see it - but where I have seen that those tend to become even more pedestrian travelled, but even the flexible post at interval type ones, I've seen most of those destroyed in the first year they'd been put in by plows, UPS drivers, you name it. It's a bit of a tall ask to put in full on curbs, as much as I think it could work. Most of our major downtown streets aren't much wider than SF's smaller streets, with parking on both sides.

Interesting points for sure. At some point I just say that sweating after a commute is something we have to deal with (same as getting fat by driving a car :-)). I'd rather bike and be sweaty than fat and driving a car (yes, I know not all car commuters are fat, but in generalities...).

The separated thing is super interesting to me. Obviously we couldn't do it on all streets, like the very skinny SF ones you talk about (I live near many of them in the Lower Haight). On those, which are not major thoroughfares, mixing bikes and cars is fine. But when it comes to a 4-6 lane street like Market St (or some in Chicago), separated makes sense. The traffic will go around if the street is less amenable to them. It's the same argument as taking down a freeway through a city. SF did it both at the Embarcardero (well, that one was by natural causes aka an earthquake), but then also at Octavia. Traffic slowed, neighborhoods revitalized. Studies have been done showing that taking these down doesn't greatly increase driving times, and often the traffic driving them doesn't actually stop in the city that they pass through. Instead they pollute the air and leave. Strong argument for taking them down, IMO.

But I've digressed. I agree there are challenges and never a one-size-fits-all solution. But there are solutions that are not currently being put into place that benefit the vast majority.

</rant>

Your association with being fat and driving is idiotic. A commute to work on a bike is not an intense activity so it's still very easy for overweight people to do. About 1/5 of the bikers I see (South Bay) are overweight. 95% of them I see commuting are not exerting themselves so they aren't getting meaningful exercise.

Sometimes the sweating just can't be avoided. The last few blocks of my ride ascend ~150 ft. It's steep and on any morning that's above 55 I'm breaking a sweat. And riding slower only drags out the effort for longer.

One of the main reasons I use my bike is because it is significantly faster than other methods of transport I have available (metro, bus, walking). Slowing down to avoid sweating negates that benefit. (Though in summer, I usually bring a spare t-shirt. Sweat itself doesn't smell, only after bacteria have had a time to act on it. Cycling to work after a shower then a change of t-shirt works fine for me - though I am not a heavy sweater).