Per million hours travelled is a terrible normalization method for comparing travel risks. The problem is that risk is usually not uniform throughout a trip.
A good example of this is air travel. Most of the risk in air travel is concentrated at the end points. The high risk parts of flight are getting on or off the ground, and flying through the high traffic area around the airport. When flying at cruising altitude between the endpoints, the risk is lowest.
Overall, a very long flight is more risky than a short flight, but if you went by risk per million hours travelled the long flight might actually appear to be less risky.
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Per million hours travelled is a terrible normalization method for comparing travel risks. The problem is that risk is usually not uniform throughout a trip.
A good example of this is air travel. Most of the risk in air travel is concentrated at the end points. The high risk parts of flight are getting on or off the ground, and flying through the high traffic area around the airport. When flying at cruising altitude between the endpoints, the risk is lowest.
Overall, a very long flight is more risky than a short flight, but if you went by risk per million hours travelled the long flight might actually appear to be less risky.