Roller Coasters are constantly accelerating and decelerating, therefore they don't have a constant speed. However, their average speed can be calculated by dividing the total distance covered by the roller coaster on the track by the total time it takes to complete one ride on the track.
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Take a video of the roller coaster as it runs along a level space of track, then measure the length of track and note from the video exactly how much time it took to traverse that length. Or, use a traffic radar gun.