I'll never forget when I met Bill Reilly. In the late 1980s, he was a big name in the development of the Highway Capacity Manual. I think he was the chairman of the signalized intersection committee. This was when Dolf May was the overall chairman. I was a newbie traffic engineer doing work out of Phoenix, Arizona for Lee Engineering.
Anyway, we had been measuring traffic flow rates at intersections in Phoenix. We had determined that the 1800 Ideal Saturation Flow Rate (The rate at which vehicles move through an intersection) was too low. But the Saturation Flow Rate was also modified by these factors that were listed in the Highway Capacity Manual. Each factor lowered the flow rate by a bit. One factor, I think it was the truck factor, really substantially lowered the flow rate. But no matter how hard I searched, I couldn't find any research documenting this particular factor.
It turned out that one year, I was to present one of my papers to the Transportation Research Board. I was excited to travel to Washington, D.C. to present the paper, but I was also excited to attend the Highway Capacity Meetings. Because I would be able to find out where the research was to back up the factors.
Well, the meeting wasn't really the right place to bring up this issue, and so after the meeting, I boldly went up to Bill Reilly and asked him where they came up with the factor. His answer surprised me, to say the least.
"We made it up", he said. "We needed a factor and we didn't really have any research, and so this is what the committee guessed". I was astonished. It had huge implications across the United States and there was no data to back it up? Amazing.
Of course I now understand that much of engineering is educated guesswork. You can't have a procedure or factor for everything. That's why experience and grey hair is really quite valuable. There's nothing that will give you that experience except monitoring and studying traffic for a career. Bill Reilly certainly did that.
We sent our research into the Signalized Committee and I think this prompted more research into the area. But there is nothing wrong with the approach that the committee took. They needed a value and they took their best guess. It was certainly a better guess than I would have made.