Can one cop ticket me based on another cop's radaring?

It's legal, but it still might give you a way to win if you go to court.

If you contest the speeding ticket and demand a trial, both officers must show up in court to make the case against you. If only the arresting officer shows up and tries to present information received from the other officer, you can object to it as "hearsay" in most states, including California. That's because the arresting officer has no personal knowledge of whether you actually committed the offense but is relying on the word of another officer.

Also, if you challenge the violation, ask for a copy of the arresting officer's notes -- usually jotted down after writing a ticket -- to see whether he wrote down anything about independently measuring your speed. For more information, see Traffic Court: Getting the Police Officer's Notes.

Even if both officers show up at trial, there are still a few standard defenses to radar tickets that you can raise with the judge:

• You might challenge the very veracity of the radar machine. For example, if there were lots of cars on the road, the broad radar beam may have picked up more than one car, in more than one lane, especially if you were far away from the radaring officer. Maybe the radar actually hit a bigger car, which was speeding alongside you.
• Also think about whether road conditions or other natural occurrences -- mountains, curves, bad weather, darkness -- might have made it difficult for the officer to get a good radar reading or otherwise keep track of you.