The crude, homemade device in the picture is actually a fairly accurate clinometer.  This one is calibrated in degrees.  It can tell you for example that Broad Street in Richmond, Virginia from 12th Street to 17th Street, that is descending into Shockoe Bottom, is inclined only about 5 or 6 degrees at the steepest points near the top.  It might seem like much more driving down it.

It can also tell you that the exercise trail in Forest Hill Park is inclined about 9 degrees at its steepest section.

When doing this sort of thing yourself remember that the sidewalk has the same slope as the road and doesn't require getting in the way of traffic.