Interestingly the system you describe, marking two points and measuring time between them is illegal in California: Definition of a speed trap: http://www.dmv.

Interestingly the system you describe, marking two points and measuring time between them is illegal in California: Definition of a speed trap: http://www.dmv.ca.gov/pubs/vctop/d17/vc40802.htm http://www.dmv.ca.gov/pubs/vctop/d17/vc40802.htm Speed traps are prohibited: https://www.dmv.ca.gov/pubs/vctop/d17/vc40801.htm https://www.dmv.ca.gov/pubs/vctop/d17/vc40801.htm