Determination of Left Turn Treatment:

If the left turn treatments are not known, they may be synthesized using the cross product of the left turning volume and the oncoming through traffic volume. Thresholds of 50,000, 90,000 and 110,000 are recommended for 1,2 and 3, through lanes respectively. The table below illustrates this calculation for the Palouse River Drive approaches:

 

Summary of Cross Product Calculations: Palouse River Drive
Approach	LT (veh/hr)	Opposing Through/Right (veh/hr)	Cross Product
Eastbound	74	409	30,266
Westbound	67	297	19,899
Northbound	119	706	84,014
Southbound	149	669	99,681

In this case, all but one of the cross products are below the threshold. The southbound left turn indicates a cross product of 99,681 which suggests that left turn protection should be provided for this movement. So southbound left turn protection will be included in the phasing design.