Hi Matt - our site is primarily designed for plottting walking, running and cycling routes, so doesn't follow the same rules as a route planner designed for plotting driving routes. We take the shortest path between two plotted points (rather than the quickest), so if the points are a long way apart, this could result in a detour away from the road you wanted to follow. To avoid this you'll need to reduce the distance between the point you plot.
I am also having this issue. I am trying to plot a rout for the NC500 this summer. The first day or two up to scotland is mainly motorways. With the 'Avoid Highways' NOT checked. To go from Wigan to our first stop in Carlisle it takes us all around lancashire when we just want the motorway/quickest route. Ideally, it should just chuck me onto M6 North all the way. See Image.
Reducing the distance between the points you plot should help. Our route planner takes the shortest route between points, as this is normally what people want for waliking, running and cycling, so this can sometimes give strange results if you plot points a long way apart.
What I meant by car/vehicle mode is using auto plot in 'Roads" mode.
Plotting in car/vehicle mode does not always allow me to plot up an interstate. It seems to try and avoid them, but the avoid highways button is NOT checked. It's very random where it wants to plot the route, throwing me off an on the interstate or highway.