Brett says that moving grain by road would double the cost for every 100 miles. He has quite a few articles on premodern farming, and may have given a different figure in other articles.
Roman roads were high tech for the era and those were still pretty bumpy. A modern road is much more efficient. Bouncing over uneven terrain not only damages the products and veg it also causes wear and tear on the cart, the animals, and the driver.
Also doubling isn’t a bad thing for the first couple of multiples. Each doubling increases the supply by a factor of four on open terrain. It’s no accident that big cities are at the confluence of trade routes. Up in the mountains creates a funnel that raises the average cost and lowers the reliability of everything too much. It’s like you’ve blockaded yourself.
Travelling by horse and cart can't be much faster than 30 miles per day. So even if you're only 30 miles away, you have a day to get there, a day at the market, and a day to get back. That's 3 days you're not working on the farm. You and the horse that pulls the cart, which of course also needs feeding, and whose day-job is ploughing.
