 The county town of Aylesbury , immediately north of the Chilterns and 37 miles ( 60 km ) from the City of London , was an important agricultural centre . As London grew , the significance of Buckinghamshire as a provider of food increased , particularly following the development of the Aylesbury duck in the 18th century . Large numbers of horses , cattle and Aylesbury ducks were herded along the roads to London 's huge livestock market at Smithfield . The strain placed on the roads by bulk livestock movements led to the introduction of a network of high quality toll roads in the area in the 18th century . The roads crossing the Chilterns followed the valley of the River Misbourne through Amersham or the River Bulbourne through Berkhamsted . These roads greatly improved travel in the area , reducing the journey time from Aylesbury to Oxford or London to a single day . 
