Multiplexing serial line
If your sensor can accommodate a "ready" line, and defer any events which arrive when it's not asserted, transmitting them when it is, your best bet may be to send a separate busy wire to each sensor and use an "AND" gate to combine the signals from all the sensors.  I would suggest that if you have four sensors, you should cycle the "ready" wires in round-robin fashion, with some "dead" time between them (when nobody is "ready"); if a byte comes in during the "dead" time, assume it was sent by the last active sensor.  Set the "ready" time long enough that a sensor will be able to react, and the dead time long enough that a sensor will have time to finish transmitting an event that occurs just before its "ready" line was deasserted.

Edit

Based upon further descriptions, given that apparently each sensor has a distinct known ID number, I'm thinking the best approach may be to have a single serial line which is connected to a PWM output via parallel resistor and diode, so that it is pulled up for 1,900us every 2ms but pulled down for 200us (if one could have a PWM switch between active-low and floating, that would be even better).  As soon as a car is seen, a sensor should start the following state machine, keeping track of how many tens of microseconds have elapsed since it began executing.


Wait for data line to be high.
Wait for data line to be low.
Clear line-low timer
Wait for data line to go high, incrementing line-low timer (and main timer) while waiting.
If timer indicates line was low less than 200us, go back to step 2
Keep incrementing line-low timer while watching for line to go low, until that timer reaches some duration which is unique for the node's ID.  If line goes low within that time, go back to step 3.
Set data line to output, transmit the data at a rate of 16us/bit or so (driving the line actively for both high and low), and start looking for the next car.  Note that the data should include the number of 10us intervals that have elapsed between seeing the car and commencing transmission.


Using a UART, this approach should allow one to process cars that arrive on any sensor, in any order, and resolve their timings within 10us, with the proviso that cars will be processed sequentially at a rate of one every 2ms, and sensors will be "blind" between the time they see a car and the main CPU gets around to processing it.  Unless there are a really huge number of sensors, that shouldn't pose a problem.  Note that the main CPU doesn't have to have any precise timing on anything but its PWM output.  Everything else can be inferred from the serial data stream (including the "long breaks" resulting from the PWM "low" pulses).