NEW DELHI: The Centre has given in principle approval to upgrade 57,500 km of state highways to national highways so as to augment the existing 1.13 lakh km of NHs across the country, Parliament was informed today."About 57,500 km length of state roads have been approved 'in principle' for their declaration as new NHs subject to outcome of their detailed project reports," Minister of State for Road Transport and Highways Pon Radhakrishnan informed the Lok Sabha in a written reply.Keeping in view the requirements to enhance road safety, reduce congestions, improve road connectivity, the Ministry has taken decision to augment the NHs network of existing 1,13,298 km, he said.He said the ministry has also reduced threshold traffic for 4-laning of NHs from 15,000 passenger car units (PCU), 11,000 and 8,000 PCUs per day to 10,000 PCUs, 8,500 and 6,000 PCUs a day for plain, rolling and mountainous terrains respectively.Decisions have also been taken to entrust 4-laning of all such NHs to the National Highways Authority of India for their implementation through additional market borrowings etc in a phased manner by leveraging limited available budgetary allocation, he said.