I am sure these topics have been discussed many times in the past on the email list and forum.
https://wiki.openstreetmap.org/wiki/Contact_channels

I would also check the Wiki, as it is supposed to represent current consensus.

Here is my opinion:

Ideally, the OpenStreetMap database would instantly change to represent current reality.

Accurately mapping physical attributes in a detailed way is relatively safe. That is, when there are enough people willing and able to keep those details accurate as they change. I know I cannot keep up, so I try to resist the urge to add things that will quickly become inaccurate. Sometimes vague can be better than confidently inaccurate.

As to mapping the intangible attributes of an object, the "Good practice" page states, "OSM is a geographic database, not a legislational database."
https://wiki.openstreetmap.org/wiki/Good_practice#Don't_map_local_legislation_if_not_bound_to_specific_objects

As is done for default speed limits, we should focus on mapping the physical attributes the laws are based on. Then the Wiki can be used to share filters data consumers can use to determine how to behave based on the law. If the law changes, one quick edit fixes all objects having those attributes.
https://wiki.openstreetmap.org/wiki/Default_speed_limits 

Some of the problem may be a British to Michigan terminology difference and iD's suggestions. A Michigander searching for "sidewalk" is suggested highway=footway + footway=sidewalk, intended for a foot-only path immediately adjacent to a roadway. A similar thing happens when searching for "bike" or "bicycle" it suggests highway=cycleway, which means bicycles only.


> Hi, is it bad to add redundant tags?

Once begun, it should be done state-wide AND be maintained.

> Although they may be redundant to people editing in Michigan, is it not better to be explicit so the router will know it is legal to bike on sidewalks?

Maybe someone should start a discussion about our use of footway on the Michigan Wiki talk page.
https://wiki.openstreetmap.org/wiki/Talk:Michigan

We could create Michigan's table for default access restrictions to change the meaning of the tags. That is, if Michigan really is unique in this among the states. However, changing the default access-es of way tag would come with its own consequences.

> I can start adding smoothness and ensuring width is present as well to improve routing (as sidewalks usually have inferior smoothness to roads, but not always, as streets can sometimes have more debris present than sidewalks [side note: is there a way to tag the smoothness of a bike lane separate from the road, for example when the road is smooth but the bike lane accumulates debris from the road?]).

Again, this is great. But it would need to be updated by someone once cleaned or repaired. As an example, I struggle to keep up with Old Lansing Road each time they chip and seal over the lane markings that designate its narrow shoulders as a legal bicycle lanes.
https://www.openstreetmap.org/way/39259913/history

> I was trying to primarily focus on adding bicycle=yes on sidewalks adjacent to one-way and especially busy/fast roads like Saginaw and Oakland to fix routing where an unnecessary (and oftentimes dangerous, as it usually means riding with traffic on a de facto 45 mph road) detour would be suggested simply to avoid using the sidewalk.

That could be helpful. I will admit I do not ride near that inner-city freeway often. When I have, I have not noticed OsmAnd or Organic Maps suggesting I ride on Saginaw or Oakland.

> I can understand the concerns about the dangers of high speed biking on sidewalks, but I would think that should be something that the router should account for when seeing the width and the combination foot/bicycle=yes. Although I think sidewalks should ideally have separate bike areas added whenever possible to make traffic safer for all road users, but obviously that's not an OSM issue.

I believe the problem is it is human nature to focus on the major roadway with heavy vehicles moving very fast, then miss the human/bicycle/scooter/one-wheeling/etc going nearly as fast just a quick turn, late stop, or backing out the driveway, away.

> As for bicycle=yes on roads, should it just be left up to the router whether that road can be biked?

Yes. Map the physical attributes (we want and can maintain), keep our state-wide law pages up to date. Then let the routers figure it out from there.

It is not just routers. CyclOsm renders roads differently based on speed, for example. I noticed this recently after mapping the signed speed limit within Evergreen Cemetery.
https://www.cyclosm.org/#map=16/42.7083/-84.5120/cyclosm

> Should I only add a bicycle tag when it is bicycle=no, such as on segregated freeways?

I would only add a tag like when there is a sign seen at the location which changes the default. For instance, the sidewalks/paths in downtown areas of many Michigan villages and cities would be bicycle=dismount. I have yet to see one that would be bicycle=no. 

> I've also been tagging roads explicitly with their foot tag, whether yes or no, because that depends on factors not related to tags on the roadway itself, but instead to whether there is an adjacent sidewalk or not. Is that okay? Should I just leave the tag blank on roads with sidewalks and only explicitly add foot=yes on roads without sidewalks?

As they do not always relate to physical attributes, the access tags are unique in OpenStreetMap. They represent the legal access, not physical, not a suggestion or judgement of the safety or practicality.
https://wiki.openstreetmap.org/wiki/Key:access

It is my understanding, by default in Michigan, as long as they are moving, people are allowed to walk on any roadway not designated as a freeway.

> I've also wondered about places where access is legal but hardly practical, such as on roads where there is no gutter area separate from traffic lanes (see the North Larch St Bridge from Lake Lansing Road to East Cesar E. Chavez Avenue and vice versa 

I tag "death canals" with shoulder=no.

https://wiki.openstreetmap.org/wiki/Key:shoulder
https://wiki.openstreetmap.org/wiki/Key:verge

> [I do now see that you marked this as cycleway:right=no. Is this sufficient to tell a router that this is not a good way to route?]).

That was likely added by me solving a StreetComplete quest. I have not looked into the details of any of the routing engines, but I am sure they would use attributes like, the roadway's classification in the network, the lack of a shoulder, lack of a bicycle lane, its max speed, number of lanes; to prefer alternate routes. OsmAnd and Organic Maps have done well by me.
https://wiki.openstreetmap.org/wiki/StreetComplete


My favorite example of how OpenStreetMap can benefit bicycling and other pedestrian uses is the work done by the fine folks in Ottawa, Canada.
https://maps.bikeottawa.ca/

Notably this is achieved with only a few elegant tag combinations, well documented on their GitHub, then shared with the community on the Wiki as they become mainstream.
https://github.com/BikeOttawa/OSM-Bike-Ottawa-Tagging-Guide