A consultant working for the Seattle City Council will present a traffic analysis today on the deep-bore tunnel that finds tolling the project will cause "significant diversion of traffic from SR 99 onto Center City surface streets." Tim Payne, the principal of consulting firm Nelson\Nygaard, says the tunnel's tolls and general traffic growth will be felt particularly hard in South Downtown. The briefing will touch on some of the worst-affected intersections:
- "This illustration depicts intersections that will experience significant new traffic compared to today as a result of three factors: general growth in the greater downtown area; shifts in traffic patterns to access Downtown; and traffic diversion that might result from tolling. The yellow dots indicate intersections that will operate at a significant level of congestion during peak traffic periods as a result of one, or a combination of, these three factors."
"The issues, left unaddressed, will impact accessibility to and the character of the Center City, particularly in the vicinity of Pioneer Square and the Seattle Center/South Lake Union areas," says a briefing paper that will be presented to council members this afternoon (.pdf). The Nelson\Nygaard report also finds that the uptick in traffic may result in longer travel times for transit and "will increase conflict between automobiles and vulnerable road users."
State and city officials say they currently lack any plan or any money to manage the flood of new traffic. Council Member Tom Rasmussen, chair of the council's transportation committee, has argued that the city has five years to make a plan, but it's unclear where the cash-strapped city or state would find the funding.
Council Member Mike O'Brien, a tunnel critic who secured $2,000 in city funds to pay for the analysis, said a few weeks ago that his some of colleagues were "in denial" about the tunnel's impacts. The council is expected to approve contracts for the deep-bore tunnel in February.
"I hope this is a starting point for the city, which is about to see a significant change in a number of historical patterns," says Payne by email today. "Managing change under any set of conditions is challenging. This will be particularly the case here, as the changes are a significant departure from the roots of what has existed throughout the development of Center City as far as auto transportation is concerned."
The state's Supplemental Draft Environmental Impact Statement found that most of the 110,000 vehicles that currently use the Alaskan Way Viaduct each day will divert to city streets and I-5. For instance, just on the southern end of the tunnel, 40,000 additional vehicle trips a day are projected use eight streets that cross South King Street
Nelson\Nygaard suggests several methods for mitigating the traffic: Dedicated transit lanes through downtown, wider sidewalks, tolling a longer segment of RS 99 to discourage drivers from siphoning off en masse at the portals, expanding tolling to surface streets and I-5, timing traffic signals through downtown, and others.
The noon briefing will be streamed live here.