PLC dummy needs help picking a device

Ok, any help would be great.
I am looking for a device to activate a speed warning sign for semi trucks. I have a device that has two 12V dc optocouplers that I need input from each to get a signal. One is a presence detector the other is a speed detector. One of the optocouplers will only give me a 1ms pulse.
I then need to control a relay (be nice if it was built in to the PLC) to give 120VAC to the flasher sign. I want to have the sign active for 7 seconds, so instead of a separate timing circuit I would like to have the PLC control the timing.
Also, to make it more fun it has to be a low temp device for minnesota weather.
Any thoughts on what device can do this.

Use those pneumatic hoses that you occasionally see on the highway. Have each of them connected to a pressure switch, and then set a bit in your PLC when the first hose gets crossed, and use that to start a timer. Reset it when the second hose gets crossed, and calculate your speed based on the amount of time it took to go from hose "A" to hose "B".

One of the optocouplers will only give me a 1ms pulse.

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That is too fast for most PLCs, so if you have to use that optocoupler, you would need a pulse stetcher.

Also, to make it more fun it has to be a low temp device for minnesota weather.

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Put the PLC and other electronics in an insulated heated cabinet, or indooors. You can run wires to the input and output terminals of a remote PLC (remote from the truck road).

Pick a PLC with integrated high speed counters. Being a Siemens user, I have used the 313C a lot for applications like these. 1 ms = 100 Hz is no problem. It is probably overkill for this application tho..