Q: Is gasoline really cheaper on the expressways?
There is a maximum gas price that is set once a month and the gas stations on the expressways (like Joban Expressway) do not increase their prices at all during a particular month. The maximum price is set by the Ministry of Economy, Trade, and Industry (METI) at the end of the previous month. You can see the price on this site. The reason for this is that the number of gas stations on the expressways is extremely limited, so drivers do not have a choice of which station to use when they need to fill up their tanks. If the gas stations could set their own prices, it would be possible for them to set unreasonable prices and the consumers wouldn’t be able to do anything about it, since the next station might be 100km ahead. (Also, cars that run out of gas often cause accidents, so it is better not to encourage drivers to shop around for the best price.)
So, this means that if gas prices went up this month as compared to last month, you might be able to get cheaper gas on the expressway than on the regular roads. However, the opposite is also true: if gas prices went down this month, the gas on the expressway will be more expensive.
The prices for regular gas in November and December were 140 yen and 136 yen respectively.
(Note that this only applies to gas stations on the expressways. Regular gas stations do not have to follow this rule.)
Source: Japan Automobile Federation (JAF) Magazine, November 2006