At a brown bag discussion this afternoon, consultants from Nelson/Nygaard will present the findings of a study about the impact the deep-bore tunnel will have on traffic downtown, and what can be done to mitigate it. The city already knows that a tolled tunnel downtown will displace about 45,000 cars onto city streets; this study breaks down those numbers by area.

According to Nelson/Nygaard's findings, "Between 16,000 and 18,000 additional daily trips will divert onto Downtown Seattle surface streets; 14,000 to 15,000 will choose I-5; and 10,000 to 12,000 will use Seattle surface streets east of I-5."

Additionally, the new tunnel will increase "conflicts" (AKA collisions) between cyclists and pedestrians and cars, and "increase demand for driving and increase vehicle miles traveled, and therefore, greenhouse gas emissions—even as improved fuel economy and low emission technology is widely employed."

Here are the impacts on one specific intersection, represented visually. (No, it's not just you. These are terrible graphic representations. Basically, the numbers you want are in the blue arrows---they show how much traffic will increase, per minute, if the tunnel is built and tolled at the highest potential level. So, for example, traffic headed southbound on Alaskan Way at King St. will increase by 23 cars a minute in the morning and 20 cars a minute in the evening.)

Nothing can totally offset all those impacts, but some of the things the report suggests include better signal timing, creating transit-only streets (!), increasing the cost of parking, buying down tolls, and improving and widening sidewalks.

Watch the brown bag live at noon here.