An interesting traffic discussion over at Rebuilding Place in the Urban Space. Richard had a set of figures on urban traffic densities, (rules of thumb), that he realised were not applicable to the project he was looking at, a freeway…
In spouting those mode capacity figures, while talking with the people who ambled by (and being questioned about the numbers!), I realized that the vehicle capacity numbers I knew weren’t relevant to the freeway part of I-66 (or the Wilson Bridge).
Not wanting to buy the Highway Capacity Manual, I rooted around the net to find the relevant data. This comes from the State of New Jersey Road User Cost Manual…
Ideal Capacity by Facility Type (Table 3.1)
Freeway – 4 lanes ————— 2,200 Passenger Cars per hour per lane
Freeway – 6 or more lanes —– 2,300 Passenger Cars per hour per lane
Multilane Highway ————- 2,200 Passenger Cars per hour per lane
Two-Lane Highway ———— 1,400 Passenger Cars per hour per lane
Signalized Intersection ——— 1,900 Passenger Cars per hour of green per lane
But, as is often the case, the comments prove as valuable as the post content. This comment in particular points out the troubles with many statistics on transportation capacities, if you can handle imperial measurements, there are some good examples of why the data can’t be right.