You mentioned building canals as a way to get Westeros out of debt. What about just building some real roads and some bridges and charging tolls (having the toll takers be royal appointments) to use them? We see how rich the Freys got by having a bridge, and if you look at a map of Westeros, the Twins is way out of the way.

Well, Westeros does have some decent roads, although as I pointed out they’re somewhat biased in favor of the capitol. 

In terms of roads, the main thing I can see is that:

• The North: Barrowton/White Harbor need road connections to Winterfell/the Kingsroad.
• The Vale: more of an Eyrie to Gulltown road? 
• The Riverlands: needs a LHT to Maidenpool Road, Riverrun to Goldroad via Stony Sept,  maybe Saltpans to Crossroads. 
• The Westerlands – Lannisport to Kayce, Lannisport to Banefort mebbe?
• The Reach: mostly needs a few spurs to connect Goldengrove and the like to the Roseroad.
• Stormlands: definitely needs a Storm’s End to Rainytown road.
• Dorne: Wyl to Yronwood, Yronwood to Godsgrace, and then Skyreach to Sandstone to Hellholt to Red Dunes.

In terms of bridges:

• Bridge over the Trident at the Ruby Ford.
• Bridge over the Mander at Cider Hall and/or Longtable? Might exist.