Why the three billion-dollar difference in benefits between comprehensive, high-resolution and partial, low-resolution strategies? The short answers are that reasonable emissions charges are too broad and diffuse, too low-resolution, and too low to divert much peak-hour traffic; and that freeways-only congestion charges are too narrow to increase speeds systemwide. Penny-a-mile-average emission charges are too loosely targeted to affect congestion. They fall lightly on all mileage, rather than heavily on the peak-hour 36% of the mileage which produces 85% of the delay. Freeways-only peak-hour congestion charges, by contrast, are targeted tightly enough to increase speeds on freeways drastically, by as much as a third. But, since surface streets are not charged in the official WSA models, only half of the diversion is to ride-sharing and off-peak hours. The other half is to surface streets, where traffic runs at 14 mph, on average, three or four times slower than on freeways. Even if the extra traffic does not slow down existing surface-street speeds, as WSA's model improbably assumed, the projected slowdown of the diverted traffic equaled or exceeded the speedup of traffic on priced freeways. Where the charge was close to optimal for freeways -- that is, about equal to the 20-30 cents per peak mile average delay cost actually imposed per peak-hour vehicle mile -- the slowdown from diversion to surface streets more than counterbalanced the speed up on the freeways and the systemwide average speed actually declined!
Conversely, very light freeways-only congestion charges made a small enough diversion to surface streets that average systemwide speed remained about the same. See WSA Travel and Emissions Modelling Summary, Final Draft, Dec. 12, 1996, Table 5. Thus, two combinations of "reasonable" emissions charges with low, freeways-only congestion charges, did make substantial emissions reductions without slowing down the system. (WSA Travel and Emissions Modelling Summary, Final Draft, Dec. 12, 1996, Table 7.) These were the combinations most favored by WSA and recommended by the Task Force's Strategy Subcommittee. They would make a worthwhile improvement over the baseline, cutting smog emissions by 5-15% without reducing average speed systemwide.