> If you build a road through downtown the same way you build a superhighway, people are going to drive on it at superhighway speeds, which is unsafe for pedestrians, cyclists, and drivers because downtown has more intersections and interactions between modes of travel and directions of travel.

Sorry, but that contradicts itself. If you build a road through downtown with the properties of superhighway (side barriers, on/off ramps, only motorized vehicles allowed, separated oncoming lanes), there would be none of said intersections and interactions between other modes of transport and the only concern would be noise pollution and more frequent merging ramps.

People don't drive at superhighway speeds only because they have wider lane markings and there is a sign posted, but because there are no oncoming cars with potential for head-on collision, because merging cars speed up almost to the same speed and you suddenly don't get a car appear sideways in front you from a side street and other similar factors (e.g. empty peripheral vision on highway vs. fast moving tall buildings downtown). If you put a sidewalk with people or 90-degree intersection on a highway, you wouldn't get highway speeds neither. Speed is not a simple function of lane width.

There is however another issue - why is the problem of safety reduced only to speed, especially when the speed is obtained by placing more cognitive load onto the driver? When the driver must constantly check the oncoming vehicle whether it doesn't swerve too far into his lane, eyeball the vehicles next to him whether they won't start changing the lanes without indicators (being closer, there is less time to use the horn or react quickly, and having less space to the another lane/oncoming lane, there is less elbowroom to avoid them) - then, yes, drivers would drive slower, but also pay way less attention to pedestrians on sidewalks, cars pulling away from intersections, parallel parked cars about to pull away and other things. Even the argument with the crossing pedestrian - yes, when you have narrower lanes, there is less distance to cover. At the same time, the pedestrian appears right in front of the cars in their lane.
Another problem related to the cognitive load and perception - you may just be shifting your problem elsewhere. If you have a city and suddenly, four blocks have narrower streets, people will slow down, because it is suddenly different and driving through takes some of their brainpower. If you have the entire city like that, then: a) it will become the norm and the speed will go up again after some acclimation, b) after a drive to the other end of the city, the drivers will be much more exhausted and likely to be involved in an accident.

Anecdata: 150-200 mile drive, one break. If you drive that on highway, in the end of the journey back in your city, you are somehow tired physically from all that sitting and paying attention, but otherwise still alert and still would spot suspicious behavior (car about to pull in front of you, pedestrian in dark clothing in the night, etc.). If you ride that on country roads (think central Europe), you eyeball every tree and bush while driving to forest (a boar or deer jumping right in front of you, common occurrence), you drive through sharp corners with who-knows-what behind them, you drive through many small cities and villages with no sidewalks and pedestrians in dark clothing. Yes, you drive way, way, way slower than on highway, but in the end, you are tired not only physically, but also mentally and you just drive home following the central lane marking ignoring surroundings to a certain level. Is that more safe?

This approach just tightens the safety of drivers to the limit, so instead of using other tricks working on the same human brain principles, you make it really more dangerous. Yes, it will make a few people catching pokemons while driving put their phone down and pay attention, but at everyone else's expense. And this is road to hell - just like the "phase out schools for talented children" discussion we had in Europe some time before - surely it will help other children in normal (slow) classes, where these children will move.

And it is the same stupid move as installing tall metal speed bump - it works, it lowers the speed, eureka! But:

- it is PITA to drive through,

- it slows down emergency services (here, we have two speed bumps right on a road from ambulance standby post, genius),

- it increases dust emissions (from brake pads, really nasty - try washing your rims from the inside), because the cars have to brake almost to stop,

- it increases fuel emissions, because the cars has to pull away again,

- drivers are trying to hypnotize the speed bump finding the best path through and in the end completely ignore the poorly marked, lit and signed pedestrian crossing behind it, not mentioning the approaching pedestrians.

Yet, you can install optical speed retarder markings, which doesn't impede ambulances, cars instinctively slow down, but doesn't have to almost stop. You can install speed-detecting traffic lights. You can use retroreflective marking built into the crossing. Replace streetlamp to better lit not only the crossing, but also the "approaching" area. But bolting a piece of metal to the road is the easiest, just like narrowing the lanes and - hey, let's ding yourselves, run over pedestrians, drive more fatigued, but you will do that more slowly!