I like the idea about the graduated movement system - perhaps..
'paths' (x2) around your cities first, and between cities no bonus to movement (but makes it easier to build roads later {thin, brown line};
'roads' (x4) between cities after Map Making {thicker, grey line};
Chivilary (? better knowledge of horses, wheels?) allows roads around cities;
'railroads' (x8) between cities after Steam Power {as is currently};
'highways', after Motorised Transport, give an extra x2 bonus to military units since citizens use railroads less with private cars, and allow non-military to use RR as above. Also a bonus to commercial {no visual change}.
Just guidelines for the movement bonuses, but a bit more evenly spread through the eras. Colonies could have RRs built to them, and get the commercial bonus after highways.
This may be way off - I just finished a 10hr shift: need
