I'm not a lawyer, so you cannot rely on this information. The usage of the photo is really the issue at hand. If you are shooting something for news or editorial, you don't typically need a release.
If you are considering using it for ANY commercial usage, you will need a release. That includes portfolios on the web, sales to third parties, etc. My understanding is that for the release to be 100% enforceable, you need to provide the person with something of value. I'm not sure of any legal precedence set for releases thrown out where that was not the case, but that is my understanding.
Editorially (news-wise) where you are shooting can influence the usage of the images, but it can also include the privacy of the person(s), too. If there is a reasonable right to privacy (even in public) then it can be problematic (think public restrooms/showers/etc.).
In the case of the motorcycle, you may also need a property release.
This only relates to the US. Other countries vary.
Again, I'm not a lawyer, this is just my understanding. If you really want the scope, check with an attorney in your state/country.