That's not specific to this road. You could probably pick any 50,000 sq km area on the planet besides Bangladesh, and the population density would be several orders of magnitude lower than that of Bangladesh, except for maybe the few largest metropolitan areas in the world. Bangladesh can't support half its population, and Canada could probably support 10x its population, so one has to conclude that the wild difference in fertility rate is not as simple to explain as a function of how much land there is or how much food can be produced there.