Measure if an approaching object stops briefly before a 'line' and if not, take a photo and upload for remote viewing

For our multi-stage mountain bike races, there is a need for competitors to stop at a stop sign on the course. Rather than have a volunteer stand at the stop sign to check that a competitor comes to a stop, it might be easier to develop an electronic solution. This stopping is necessary to ensure all competitors start from the same location. This electronic solution primarily needs to: - measure that a rider is less than 100cm from the sensor (stop sign) - measure that a rider holds their position for 5 seconds (i.e. their distance does not decrease more than 10cm for 5 seconds) - confirmation by a green LED that if a rider has held their position for 5 seconds, they are permitted to proceed - the solution determines that the rider has proceeded (eg their distance becomes say 30cms or less?) or even another ultrasonic sensor aimed perpendicular to the race track to act as a 'virtual breaker beam' and if this sensor detects movement across it, then a rider has proceeded. - if a rider proceeds before the green LED confirmation, a photo is taken and uploaded to the cloud for access at the race village so we can apply a time penalty to their results - once the above has occurred, an LED turns on after 30 seconds - one the above has occurred, an LED turns on after 60 seconds Their might be a little 'trial and error' of the provided times but I assume the coding could easily be modified after a testing phase. Hardware: - I have several Arduino boards and batteries from an unfinished project that I could use and we would need to add the ultrasonic sensors and camera. However, it may be easier if we used cheap mobile phones to act as the platform with their onboard camera and internet connectivity but I am guided by your advice. If mobile phone based, the green LED could simply be the screen turning green and then a 60second timer shown on screen. - The most affordable sensors I have found would be these [login to view URL] and picture attached - but I am open to suggestions. If we use a mobile phone platform, these sensors may somehow have to be connected via bluetooth or usb? However, these units will need to be on for a minimum of 5 hours I am looking for someone to offer a good solution and other ideas are welcome from my suggestions above. I will need to make 8 of these units so construction time is important followed by cost. It is also important to identify at which stop sign the photo was taken (which could be just code based and added to the filename of the photo eg [login to view URL]).