plc based speed indicator

i have an idea to implement a system for indicating speed of vehicle. but i will not measure the speed of any vehicle directly. i will place two photoelectric sensors and the distance between them mayb 1 meter or 2. when any vehicle reach at the first sensor, it will sent a signal to the station where the sensors are connected with plc ladder logic. when the signal from the first sensor arrives, it will turned on a timer. the timer mayb set to a specific duration, mayb for 10 sec. this 10 sec is the maximum allowed time for the vehicle. any vehicle may reach the second sensor after 10 sec but thats not an problem because that vehicle mayb a slow speedy. but if the vehicle reach the second sensor before 10 sec then it will indicate that the vehicle is in high speed so it will sent an alarm signal at the station. can any1 pls help me for designing the ladder logic for this problem?

Welcome to the forum.

it would help if you tell us what plc.
Sounds like homework.
You are already on your way just by telling us what you need, so here's some help.

1. vehicle makes sensor and starts timer.
2. vehicle goes past sensor and keeps timing.
3. vehicle makes 2nd sensor, store the time and reset timer for next vehicle.
4. if stored time >= 10 sec, do nothing and reset stored value.
if stored time < 10 seconds, sound alarm

regards,
james

i'm working with LOGO siemens PLC.
i think you got the clear knowledge of my idea. but i dont know which timer could be useful for this purpose. and it will be so helpful for me if you sent me some rough ladder diagram using that specific timer. thank you.