My arduino project was installed on the revolving doors of the University Center. Basically, it uses an accelerometer to compute the speed of the doors as someone pushes it. The LED display then reports back the rank of the person out of the total number of people who passed through when the box was installed.
Adafruit ADXL345 – Triple-Axis Accelerometer
Adafruit 0.56″ 4-Digit 7-Segment Display w/I2C Backpack – Red
Read more here: cmuems.com/2013/a/michelle/11/11/how-fast-can-you-spin/