>This doesn't work:
But this one does although possibly not in way you would expect. Your time string has a space preceding the 8. Include that space and rows are returned.
SELECT EventTime,
Count(EventTime) AS count
FROM trafficdata
WHERE EventTime between ' 8:%%:%% PM' and '10:%%:%% PM'
GROUP BY EventTime;
The preceding comments about using proper date and time formats are a much preferred solution