Earth Log

Earth Log: One more time -- how did they figure 110 million miles a day?

I heard from enough people. So I did it again. I asked how the state calculates the 110 million miles that are driven daily in the San Joaquin Valley.

Perhaps unwisely, I wrote about all those miles last week. The 110 million miles per day citation is in the state's 2013 air-quality almanac of emissions. I thought it might be a nice way to get your attention to discuss ozone.

We've had this conversation before. Years ago, I asked the state to explain a similar figure in a previous air almanac. After fumbling my way through a wave of technical jargon, I had hoped never to go there again.

OK, enough stalling. Here's how the state makes this 110 million miles of sausage:

Looking at the San Joaquin Valley, state officials combine information on population, employees, households, work travel, non-work travel and household trend surveys. They add in the entire roadway network.

They also use traffic counters. Those little dark hoses you run over in the roadway are counting. Turns out, these smart hoses can distinguish the weight of the vehicles, sorting out big trucks from little cars.

Of course, sophisticated mathematical models and computers use all this stuff to come up with the 110 million miles.

I know what you're thinking. What if we counted every mile driven on every road? We would count all day and all night in all eight counties of the San Joaquin Valley.

And what if we threw in all the traffic passing through on Interstate 5 and Highway 99?

What if we somehow knew about all the little drives everyone makes to pick up milk or Starbucks coffee or the kids at your aunt's house?

Would it really add up to 110 million miles in one day? As I said many years ago, I have no idea.

I feel the same way when federal scientists explain how much value is placed on a human life and then use the value to arrive at a number for the economic impact of early mortality due to air pollution.

Engineer Jonathan Taylor, chief of Transportation Planning at the California Air Resources Board, walked me through the mileage stuff. He made a convincing case for the 110 million miles.

It still seems incredible, I know. My only frame of reference for 110 million miles is the distance between Earth and the sun -- 93 million miles. And I've never taken that trip.