No trespassing signs have legal weight with certain conditions, and it's up to the judge. I've been in a court case where simply taking pictures outside of the property and showing that the gap between each no trespassing sign was more than 100ft wide was enough for a judge to throw out the charges. I was dirt biking on the power company's property, you see. It hinges on the defendant not noticing such restrictions. In both cases, the robots.txt meant jack. Even their security measures usually meant jack. Instead, it was a legal cease and desist from a lawyer that constituted "no authorization."