Unfortunately it would also accelerate the decline of the railways.While spending on trucks for army would certainly boost motor output, investments on large-scale motorway construction, as you said above, would raise the industry both qualitatively and quantitatively by boosting the production of higher performance motor vehicles.
Replacing horsed transport (HT) with mechanical transport (MT) earlier would save the Exchequer money because lorries were cheaper to run than horses. I put some cost comparisons that proved it in another thread which I found in the Army Estimates.