A lot of times, services run under the blvd. beside them, or power lines above them. Also, often times, in old neighberhoods, there is a lot of trash under them including old coal cinders, which also cause a lot of interference.
Around here (Houston Texas) they put down what looks similar to a metal fence material down first and then pour the concrete. If you use a small coil on your AT you can get pretty close to the edge of the sidewalk with it. I work the sidewalks by walking towards the sidewalk up as close as I can get and then stand on the sidewalk and work my coil from the other direction walking away from the sidewalk.
Any decent detector will sound off good on or near sidewalk slabs because of the iron mesh reinforcement. If you want to work close to the sidewalks, just put the sense at min and work as closely as you can without picking up the iron. Problem solved. Good luck mark...
Most commercial property and public property sidewalks will have rebar or wire mesh in them. If you hold your detector close them rather than wave it your detector will quiet down. Along sidewalks or parking lots can be a treasure trove. I found over $2.50 today around the perimeter of a concrete basketball court.
You will get used to false signals and it may sound like a coin but will read under 40. If you switch from pro to std you will notice a difference along a sidewalk.
You have to try lots of different settings, even iron audio. Even if you think your getting interference from the sidewalk if there is a real target along it will ring different and you will know it.