I like to commute by bike but I'm sure it's not always viable. It would depend on several factors. In a big city commuting by bike is usually faster than anything else: distances are shorter - I ride 8 miles each way, few people ride more than 10 miles - and traffic makes driving unbearably slow during rush hours and mass transit is unreliable.

So as long as my other commuting options are not significantly shorter and more convenient I'd ride a bike. Let's say it'd take 30 minutes by subway and 1 hour by bike I'd still ride a bike. If I could drive to work in 10 minutes (and have a free parking spot LOL) versus 1 hour by bike, I'd probably couldn't resists and would drive, so I'm glad that's not the case

In rural areas commute distances are longer and driving is usually quicker so my cutoff would be probably be 20 miles each way in flat area and in hilly areas probably 10 miles each way. Also the quality and safety of the road would be a big factor.

Also, if my job provided a shower facility, lockers, etc. I'd opt for longer commute by bike.

The ride is supposed to be fun. If it becomes frustrating and dangerous I wouldn't ride to work.

Adam