While I only do this sporadically (since I don't do a lot of street photography), here's my suggestion for the problem of model releases...and it's a problem most people photographers have.
Whenever I photograph someone I don't know, I ask them to write their email address ( and sometimes phone number) in my notebook, with a promise to email them the picture if it looks good. I then photograph the notebook page so that, when I'm looking at the pictures, the email address for the person will be right after any pictures I've taken. I can then easily send them the image and I can also contact them if I find myself in a situation where I need a release. I also give them my card when I'm talking with them - might as well do a little self-promotion.
Getting releases from everyone is time consuming, and unnecessary if the picture doesn't get used. But by getting email addresses, the interruption is short and not particularly threatening. Asking them to sign a model release full of legal jargon is intimidating both for you and for them. I know I wouldn't sign one. Why should I expect anyone else to sign one.
For some photographers, getting a signed release is probably easy. But I'll bet it's not a really transparent and honest situation. I'm a believer that anyone who signs a contract shouldn't be tricked and should know what they're signing. If you actually told people that what they're signing gives you the right to sell their picture for any use whatsoever, without further authorization from them, and with no compensation to them, who'd sign something like that? People do, but probably because they really don't understand what they're signing, or they, for some absurd reason, trust the photographer. ;-)
Finally, I have found that the act of photographing the notebook page with the email address always impresses people with my ingenuity. Because it is, after all, brilliant.
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