I always used a small size 6 foot retractable leash carabinered to my belt on the right side. The weak spring keeps the leash out of legs and wheels and dosn't pull my girl into me, and i can reach one hand down to lock it keeping her "on wheel" for traffic areas.
I've ridden in the city, the multi use trails, slow and with my dog at a dead run. It's a great way to get a running dog some exercize. It took me a couple months to train her not to run away from the bike, she never goes near the front wheel. (And she was a few years old when we started so 'old dogs...'
I miss my dog. sniff...