<P> The first keep - right law for driving in the United States was passed in 1792 and applied to the Philadelphia and Lancaster Turnpike . New York formalized RHT in 1804, New Jersey in 1813 and Massachusetts in 1821 . </P> <P> Influential in Europe was the 1920 Paris Convention, which advised driving on the right - hand side of the road, in order to harmonise traffic across a continent with many borders . This was despite the fact that left - hand traffic was still widespread: in 1915 for example, LHT was introduced everywhere in the Austro - Hungarian Empire . However, three years later the Empire was split up into several countries, and they all changed eventually to RHT, notably including when Nazi Germany introduced RHT with almost immediate effect in Czechoslovakia in 1938 - 39 . </P> <P> Sweden was LHT from about 1734 to 1967, despite having land borders with RHT countries, and approximately 90 percent of cars being left - hand drive (LHD) vehicles . A referendum was held in 1955, with an overwhelming majority voting against a change to RHT . Nevertheless, some years later the government ordered a conversion, which took place at 5 am on Sunday, 3 September 1967 . The accident rate dropped sharply after the change, but soon rose back to near its original level . The day was known as Dagen H ("H - Day"), the' H' being for Högertrafik or right traffic . When Iceland switched the following year, it was known as H - dagurinn, again meaning "H - Day". </P> <P> In the late 1960s, the UK Department for Transport considered switching to RHT, but declared it unsafe and too costly for such a built - up nation . Road building standards, for motorways in particular, allow asymmetrically designed road junctions, where merge and diverge lanes differ in length . </P>

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