It is more a political question than an engineering one. It is no great effort if you can already make a road to use simple physics to keep it straight, as with buildings from times BC which were also built with very accurate lines, (eg the pyramids, American Indian monuments). In Britain, the Romans had the universal power to build roads wherever they liked as they basically took over the land as invaders and had no regard for who owned the land. Internationally, even the poorest countries tend to build straight roads, it's the obvious way if not interrupted by physical features to traverse, but as Britain stopped building straight roads except for rarities after the Romans they stand out. Due to our major land-owner's great economic power, they have prevented use of their land for roads since the Romans lost power, and it is the only country I can find that doesn't have straight roads as the norm. For instance, even Baghdad on the news showed a tendency towards a grid system. Other capitals like Paris and Brussels were planned unlike most cities in the UK, and you'll have to go to Glasgow or Middlesbrough to find ones in the UK. Too much information?!