Optimizing transit routes using Bing API

Hello,

I am working on a problem where I have to solve travel distance for people that drive and also use public transport. I have created the following function that does solve the problem but I am looking to make sure that there is always a consistent answer (i.e it always uses the same route for public transport, whatever time or day it is).

I'm not sure the highlighted parameter is having the desired affect and wondered if a) I am using it correctly and b) there is a different way to do it.