For my husband and sons, however, the answer is easily 3-5 miles for essential needs, because they are all cyclists and thus have the endurance and the street cycling skills I lack. They're also less worried about biking in traffic.
Honestly, learning how to bike with traffic was the biggest factor in increasing my willingness to bike. It's like a whole other world, like Pleasantville's transition from grey scale to Technicolor.
Before: biking was nice with the fresh air and the fun coasting down hills, but I was less willing to go to places where I would need to leave bike paths, I had squeeze passes all the time, I had to have my head on a swivel to cover all the traffic lanes and entrances because I didn't know where the next unpredictable motorist was coming from. Even with all that, I knew biking was fantastic and made me happy.
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After: I went from able to bike to being empowered to bike. I can go anywhere in my town, even if I haven't memorized the road conditions because I now know how to interact with the road in the safest manner. I never knew how much anxiety I was experiencing until it wasn't there any more. Learning how to bike with traffic error-proofs out the main collision types so it's so much safer. It went from close calls almost every day to
maybe one close call a month. Since you're more easily seen and you see more, you're head isn't on a swivel, you know where to look for collision possibilities and prevent them. Granted, it did take me the better part of a year to internalize that my safety was more important than a car's convenience, that's how ingrained ride-as-far-right-as-possible* was.
Seriously, can't recommend it enough.
Some places to start:
https://www.bikexprt.com/streetsmarts/https://www.bikeleague.org/content/ride-better-tips https://www.bikeleague.org/content/where-should-i-ridehttps://cyclingsavvy.org/road-cycling/https://vimeo.com/iamtraffichttps://iamtraffic.org/http://ipmba.org/resources/instructor-resourceshttps://www.sheldonbrown.com/commute.html (it's not glamorous, but Sheldon's site is a great source of info. RIP.)
Some handy visuals:
https://cyclingsavvy.org/lane-width-space/ (scroll down for the interactive bit)
https://cyclingsavvy.org/wp-content/uploads/2018/09/intersection-crashes-and-prevention.jpghttps://cyclingsavvy.org/wp-content/uploads/2018/04/How-wide-he-thinks-his-car-is-1030x756.jpghttps://cyclingsavvy.org/wp-content/uploads/2018/12/CVC-21202-flowchart-handout.pdf (this mirrors most state laws)
But to answer your question, here is my current life (one way distances):
Grocery: 1mi
Downtown/errands: 3mi
Work: 6mi or 3mi depending on that week's building
Terrain: hilly-ish, ~100ft elevation changes
Weather: all year, winter is the best since drivers are going slower and it is quite obvious why I'm not huddled on the shoulder (it's full of snow). I do the mountain bike with a backpack with yellow cover.
My normal gear: gravel-type road bike with rear rack & panniers. Current load max: 48lbs. I did the backpack thing for the better part of a decade, panniers were a game changer, another Pleasantville moment.
Glad you're thinking about getting him a better bike, it'll make all the difference.
Happy riding!
*One little word can influence so much. No law says "as possible", they all say "as
practicable with exceptions".
**All of this is USA centric
***Anyone know how to remove the embedded videos? The bbcode doesn't show anything for those two url lines.