It is simple: too much demand for available capacity results in congestion.
Solutions are: reduce demand i.e. traffic, (unlikely in Harrogate, it will almost certainly significantly increase), or increase capacity, i.e. more roads. Increasing the capacity of existing roads is not possible based on recent experience. We are therefore left with the option of increasing capacity with new roads.
As previously stated the successful planning of the new roads is essential. They must take traffic congestion and channel it to new roads that will be free flowing.
That The Campaign to Protect Rural England identifies some failed by/pass/relief roads (if they are to be believed!), does not mean it is impossible to design successful ones. So it is up to CounMackenzie, NYCC and HBC to ensure that any bypass/relief road, their tributary roads and connection points are effective. Otherwise it is money down the drain.