Famous US political journalist and commentator, Michael Kinsley, writes in the LA Times about the SR520 bridge toll that I reported on last year here (noting the congestion pricing that has since been applied), with this interesting statistic:
Since the toll was imposed, traffic on the toll bridge has dropped about 40%, while traffic on the free bridge has risen 10%. Overall traffic to and from the east side has dropped about 6%.
Of course, the main point of his article isn’t about the traffic or economic implications about it, he’s being philosophical about it as an example, saying it makes society less egalitarian because people can pay to avoid congestion. I’m not going to engage on that, except to say that I’m sure he’s flown in premium cabins on airlines happily bypassing queues at airport check-in and boarding, and people pay for TV, pay for valet parking, pay for the best seats in theatres etc, and he doesn’t seem to mind that. Indeed, the SR520 toll is to help pay for the replacement bridge.
The interesting result is that a 40% drop of traffic on one bridge (with two lanes each way) has resulted in only a 10% increase on the other bridge (with three lanes each way excluding the reversible two HOT lanes on the I-90 bridge), resulting in a 6% overall reduction in crossings. The report I blogged about last year predicted a 50% reduction.
What this demonstrates is that the toll has made more motorists think about whether driving is the correct option (I don’t have figures for public transit use) or whether people consolidate trips, travelling less when faced with paying the cost of the infrastructure they use. In either case, it shows that tolling one route whilst leaving another free does not just result in a 1 for 1 diversion, it does mean that there is some trip suppression and some mode shift.
As I said before, what's particularly interesting is that congestion pricing has been introduced on the SR520 bridge. The effect of this means different patterns of behaviour change, with more mode shift and trip suppression likely at peaks than other times (and no toll overnight).
For cities considering tolls on one crossing but not another, it may be an interesting case study because the automatic concern will be gridlocking the other bridge. In this case, the other bridge also has a tolled alternative in the form of express lanes. The simple response to congestion concerns is that users of either crossing have uncongested options, and a free one. Washington State has introduced tolls on SR520 for financial reasons, but has cleverly designed it to also maximise efficient utility of the crossing as well.