Ffordd Pen Llech is a minor road in the town of Harlech which lies within Snowdonia National Park, North Wales. It is the steepest signed public tarmacked road in the United Kingdom and the steepest street in the world, with a gradient of 37.45%.
Ffordd Pen Llech is one of two roads surrounding Harlech Castle World Heritage Site, and linking the higher town centre with the Cambrian Line railway station, housing and camping areas close to sea level. Its descent of the rock spur to the north of the castle gives it a tangentially measured gradient at its steepest section of 1:2.73.
Whilst this translates to the vertical rise being 36.63% of the horizontal going, it is normal practice for UK highway authorities to round gradients to a nominal figure to avoid confusing road users with excessive precision; hence the warning sign gives a slope of 40%. Previously, in common with all earlier gradient warning signs in the United Kingdom, the sign displayed an arctan ratio of 1:2 1⁄2, and was subsequently changed to the new standard of the tangent expressed as a percentage.
The street is a single-track road for the majority of its length. To avoid problems with vehicles meeting on the slope and being unable to restart, the northern half was made a one-way street for the main descent.
According to en.wikipedia