Minority voice here. I've ridden with a Doberman on leash.

Vrka & I loved it, but I don't think I'd try the experiment with just any dog. Obviously, the dog has to be able and willing to Go Places beside the bike. The dog also has to be well enough trained to not be (too much of) a hazard. Vrka was both long-legged and the most trainable dog I've ever known. My current mini Dachshund is not a good candidate.

Here's what I did: I trained her to heel on the wrong (right) side. No, it wasn't on purpose, it just sort of happened, but it worked out well because I then had me between the dog and traffic when she was on heel. No darting out in front of a semi. (Yes we rode for a short way on a highway.)

I bought a harness for her so that she could pull me up hills without strangling herself (I was much thinner then). I think it took twice before she knew what the harness meant. There is nothing like a Doberman grin when you are about to do something they love. I already had a long leather leash.

I think I was more unsure on the initial ride than the dog. I just got her, me and the bike in position, said "Heel" and off we went. It really was no big deal. The cue for her to enjoy herself and run ahead was "OK." If I needed her beside me, I'd tell her to Heel and she'd drop back beside me. As I said, a well-trained dog makes this possible.

No, this wasn't Serious Bike Training. We didn't set any land speed records, but we had a great time. We took breaks to play in the river (she'd retrieve skipping rocks from the bottom), share picnics, all that.

Mountain biking would have been more interesting with that dog. I had to keep in visual contact with her, as she was really bad at placing sounds. I'd call her, and she'd start running to meet me in the wrong direction. This was often a problem.

So yes, biking with your dog is possible, but you really need the right dog.