I need to use google trends data in a commercial project. Does the google trends license prohibit free usage?
From what I can see in the Terms:
Using our Services does not give you ownership of any intellectual property rights in our Services or the content you access. You may not use content from our Services unless you obtain permission from its owner or are otherwise permitted by law.
The short answer to your question is likely no. It is possible that you may have fair use rights to it, though this is unlikely if you are using it for commercial purposes.
From their Terms titled: About Software in our Services:
Google gives you a personal, worldwide, royalty-free, non-assignable and non-exclusive license to use the software provided to you by Google as part of the Services. This license is for the sole purpose of enabling you to use and enjoy the benefit of the Services as provided by Google, in the manner permitted by these terms. You may not copy, modify, distribute, sell, or lease any part of our Services or included software, nor may you reverse engineer or attempt to extract the source code of that software, unless laws prohibit those restrictions or you have our written permission.
Do not sell or lease a product using Trends information (you could possibly sell a program that connects to the 'API' though).
If I were in your position, I would contact and IP Attorney or email google.