Externals

External trip making in SeaCast consists of three distinct trip types:

• Trucks
• Work Trips
• Non-Work Trips

External trips are an input to SeaCast and are balanced to observed vehicle volumes across 18 external stations in the region. Traffic volumes at external stations are updated with each new base year and are obtained from the Washington State Department of Transportation. 80% of the total non-truck external trips are assumed to be SOV and the remaining are treated as shared ride trips. The total daily external vehicle trips in the 2014 base year are:

Total Daily External Truck Trips

TAZ	Location	Daily Count
3733	I-5 @ Thurston County	124,000
3734	SR-507	20,000
3735	SR-7	2,350
3736	Paradise Rd.	2,100
3737	SR-123	570
3738	SR-410	500
3739	I-90	29,000
3740	US 2	3,250
3741	Mountain Loop Hwy	90
3742	SR-530	1,500
3743	SR-9	2,000
3744	I-5 @ Skagit County	55,000
3745	Old Pacific Hwy	9,100
3746	SR-532	20,500
3747	Mukilteo-Clinton Ferry	6,000
3748	SR-104 (Hood Canal Br.)	16,300
3749	SR-3	16,800
3750	SR-302	4,100

External Truck Trips

Truck trips are based on a combination of observed truck counts where available and HPMS based truck percentages for lower volume screenlines where truck trips do not exist. External truck destinations continue to be based on the REEBIE truck flow data from the early 2000's. These destinations will be updated using currently available ATRI data in the near future. External truck trips account for approximately 28,000 of the daily external vehicle trips (9% of the total external trips).

Total Daily External Truck Trips

TAZ	Location	Daily Count
3733	I-5 @ Thurston County	11,800
3734	SR-507	1,000
3735	SR-7	200
3736	Paradise Rd.	200
3737	SR-123	0
3738	SR-410	0
3739	I-90	5,100
3740	US 2	200
3741	Mountain Loop Hwy	0
3742	SR-530	400
3743	SR-9	100
3744	I-5 @ Skagit County	5,000
3745	Old Pacific Hwy	800
3746	SR-532	700
3747	Mukilteo-Clinton Ferry	200
3748	SR-104 (Hood Canal Br.)	900
3749	SR-3	700
3750	SR-302	200

External Work Related Vehicle Trips

External work trips have been revised to use the US Census Longitudinal Employer-Household Dynamics (LEHD) datsets to assign external work trips to the region. These external work trips make up approximately 70% of the 313,000 daily vehicle trips that enter the region each and every day. These trips are a direct input to SeaCast and absorb a share of the jobs available in any given parcel. These distributions do not change into the future however the level of external activity is grown based on forecasted employment growth in the region. This adjusts total external trips over 45% between 2014 and 2040.

Total Daily External Work Vehicle Trips

TAZ	Location	Daily Count
3733	I-5 @ Thurston County	87,700
3734	SR-507	15,000
3735	SR-7	1,500
3736	Paradise Rd.	1,600
3737	SR-123	0
3738	SR-410	300
3739	I-90	19,300
3740	US 2	2,400
3741	Mountain Loop Hwy	0
3742	SR-530	1,000
3743	SR-9	1,000
3744	I-5 @ Skagit County	41,700
3745	Old Pacific Hwy	4,400
3746	SR-532	11,000
3747	Mukilteo-Clinton Ferry	5,200
3748	SR-104 (Hood Canal Br.)	11,200
3749	SR-3	13,400
3750	SR-302	2,700

External Non-Work Related Vehicle Trips

External non-work trips are calculated by subtracting the total truck and work trips from the observed external vehicle count. Non-work trips are then distributed using a gravity model formulation for non-work based trips.

Total Daily External Non-Work Vehicle Trips

TAZ	Location	Daily Count
3733	I-5 @ Thurston County	24,500
3734	SR-507	4,000
3735	SR-7	600
3736	Paradise Rd.	300
3737	SR-123	500
3738	SR-410	100
3739	I-90	4,600
3740	US 2	600
3741	Mountain Loop Hwy	100
3742	SR-530	100
3743	SR-9	900
3744	I-5 @ Skagit County	8,300
3745	Old Pacific Hwy	3,900
3746	SR-532	8,800
3747	Mukilteo-Clinton Ferry	600
3748	SR-104 (Hood Canal Br.)	4,200
3749	SR-3	2,700
3750	SR-302	1,200