Between 2003 and 2006, the most improved safety record was for the A453 from the A38 to Tamworth in Staffordshire. This rural single carriageway saw an 88 per cent drop in the number of fatal or serious collisions in the last six years, taking it from a medium risk road to one of the safest. According to the Foundation, this has been achieved by introducing traffic lights, speed limit reductions and village pedestrian facilities. Research undertaken in July 2008 has shown that investment in a safe road infrastructure programme could yield a one-third reduction in road deaths, saving as much as £6 billion per year. A consortium of 13 major road safety stakeholders have formed the Campaign for Safe Road Design, which is calling on the UK Government to make safe road design a national transport priority. The earliest evidence of engineered roads dates back to the 1st century BC. A metalled and cambered road, 1.5 metres high and six metres wide, was unearthed at Bayston Hill quarry, near Shrewsbury. A timber road was preserved in peat in Geldeston, Norfolk, with tree rings suggesting a date of 75 BC, probably built by the Iceni tribe. Roads built in the first phase of Roman occupation (43–68 AD) connected London with the ports used in the invasion (Chichester and Richborough), and with the earlier legionary bases at Colchester ("Camulodunum"), Lincoln ("Lindum"), Wroxeter ("Viroconium"), Gloucester and Exeter. As Roman influence expanded, so did the network, until around 180 AD when the known network was complete. Few Roman roads extended into Scotland due to their inability to subjugate the local population. Part of the Scottish Lowlands came under Roman control in 142, and the Antonine Wall was constructed on the northern boundary. However, the Roman legions withdrew in 164 to their former northern boundary, Hadrian's Wall. The primary function of Roman roads was to allow the rapid movement of troops and military supplies, but it also provided vital infrastructure for trade and the transport of goods. The roads were paved, a first for the island, and could carry heavy goods in all weathers. Following the Roman withdrawal from Britain, road maintenance became a very ad hoc activity. A network of roadways was developed in Britain in the Middle Ages to supplement the use of rivers as a system of transportation. Many of these roadways were developed as a result of trading of goods and services, such as wool, sheep, cattle and salt.