Hills or Curves
On hills or curves target direction (with respect to radar) is changing, this causes the Cosine Effect angle (alpha) to change. A changing Cosine Effect angle results in measured target speed changing, the faster the angle changes the faster measured target speed changes (acceleration or deceleration component). If measured speed changes too fast the radar misses (does not display) target speed.
Cosine Effect Due to Hills / Curves
Note the above figures illustrates targets on a hill (side view), or a curved road (top view). Alpha is the Cosine Effect angle, d is radar (closest) distance from target path. The steeper the hill or the tighter the curve the greater the angle alpha, and the greater the measured speed error and the greater the acceleration component. Moving radar introduces another component that generally increases the target acceleration component for approaching or receding targets, and decreases for same-lane targets.
The below figure is a graphical representation of the Cosine Effect for measured speed, as a percentage of true speed versus angle (alpha) between radar and target -- the larger the angle the larger the error and the lower the measured target speed. For example at angles of only a few degrees the measured speed is 99 to 100 percent of actual; at an angle of 60 degrees the measured speed is half (50 percent) the actual target speed.
Angle vs Measured Speed (percent of actual speed)
Just because the Cosine Effect works in favor of the motorist (moving mode radar has an exception) does not mean one cannot become a victim of the Cosine Effect.