Metric conversion of the roads would be something done with a long period of planning and a short period of conversion. The Government would decide to start producing road signs with metric-only measures, placed at metric distances, but delivered with stickers for imperial measurements. Over the long term, road signs would be swapped over and ready with a minimum of effort and expenditure. Then, once a sufficient amount of them have been done, you have a weekend during the summer when people go around removing the imperial stickers. Road signs are primarily located where people already live or travel, and removing stickers is not a fantastically difficult process, so you could easily convert have tens of thousands of people going around converting millions of road signs in a couple of days. Motorways and trunk roads tend just to have national speed limit signs, so there's no need to change anything over here, and any modern digital road signage would be trivial to convert to show metric measurements. Converting cars wouldn't be incredibly difficult as the car manufacturers already have to produce RHD metric speedometers for the Republic of Ireland. They would be delighted for the roads to go metric as it means reduced costs for lower-end cars with analogue gauges, while higher-end cars with digital gauges can already be converted with the push of a button. Modern trains use digital speedometers, which are also necessary for the new signalling technology that is being rolled out across Europe, so there's no extra cost involved there. Even steam trains will need to have these fitted to continue running on NR rails, and there would be no great difficulty in dismantling their original speedometers and replacing the imperial numbers with metric ones. You have to remember that the current situation is the worst possible one, as the roads involve both metric and imperial measurements. Lorry drivers have to record the distances they travel in kilometres, and we have to buy fuel in litres, and we measure carbon emissions in grams per kilometre, yet we record distances in miles and yards. The cost of conversion would be negligible in the long term, yet the costs of not converting will linger every year in lost trade and reduced efficiency.