With those constraints, the impact on culture is amazing. Because it's so difficult to get from one place to another, each town has it's own character. Japan has kind of a difficult geography and the towns are forced into the valleys between the mountains -- usually the towns are all connected, because there is limited space. But, there are 4 towns in the 25 km between my town and the nearest big city and each one is completely different. Each town has their own grocery stores, restaurants, bars, clothing stores, etc, etc. Not only that, but each town has it's own specialties -- for example fish if it has a port, fruit because it is on the south side of the mountain, tea because it is on top of a flat plain with plenty of sun, etc.
When I lived in Canada, each big city was this 60km diameter disc of monoculture. Just suburban sprawl, designed to enable each person to have a 2 car garage and a swimming pool in the back yard (which you can only use 2 months of the year). It's only when you get into the older parts of the city that you start to get cultural variation and interest.
Although it seems counter productive, I'm a big fan of making it difficult (or at least expensive) to travel quickly from place to place. I like self contained communities where there is a reason to stay in the community, to meet people, to encourage small businesses, etc.
It's a foreign concept to many people, though. There is a kind of attitude of, "If I can have it all, why is that worse?" If there is a great restaurant 60km, why would you want to stop yourself from being able to get there? The more range you have, the easier and cheaper it is to get places, shouldn't that mean you have more options? But I think the reality is that such mobility only serves to consolidate resources and rob you of diversity. The great restaurant that might have sprouted in your neighbourhood will never get off the ground, because people will simply never give it a chance -- they would rather drive the 60 km for a known quantity.
(I have to say that 15 kph would bother me, though, as an avid cyclist -- hard to get a good work out at that speed ;-) ).