How much of that swamp, marsh, and wetlands was drained in order to enable/improve farming and forestry? To me this has less to do with living patterns and more to do with commercial exploitation of land. I am unsure how much of a reduction in draining we would see if we remove cars and roads from the equation.
Looking at Sweden, there was a major movement in the last century to drain wetlands in order to improve timber production. This forestry policy is argued to currently produce about 1/4 of all green house gases released in this region, and a major cause to lower ecological diversity.
Much, much more. Industrial-scale agriculture is by far one of the most devastating things that humans do to the environment; my focus on roads wasn't meant to imply otherwise.
(The larger observation is that, while we use more land for agriculture than we need, there is at least a prima facie argument that industrial agriculture feeds our planet. Whereas a 6 lane highway through a drained wetland to a low-density suburb does very little for our planet.)
Not all agriculture even feeds the planet. E.g. cotton grown in the central valley of california makes its way to mexico to become budget metal spring mattresses. Does the world need more metal/wood/cotton boxes that end up on the side of the road at some point in their lifetime? State economic planners seem to think so at least.
A lot of farming would not be viable without roads. It's something of a blind men and the elephant question. Roads weren't originally developed for cars; the US Constitution prescribed the authority to build roads for the postal service. Roads, especially paved roads, make it possible to send fresh produce to markets in cities hundreds of miles away, to collect grains at mills, and to distribute and maintain farm equipment.
In warm regions, wetlands were also drained to control malaria, particularly in Brazil and the American South.
Looking at Sweden, there was a major movement in the last century to drain wetlands in order to improve timber production. This forestry policy is argued to currently produce about 1/4 of all green house gases released in this region, and a major cause to lower ecological diversity.