Park Street Roundabout is a major junction in Hertfordshire between the A405, A414 and the former A5 Watling Street, now numbered A5183. It used to be junction 1 of the M10, but the motorway was downgraded in 2009 to A414.
It is also well known for being the example roundabout in the Highway Code, although the place names and road numbers are altered.
The roundabout was first built in the 1930s as part of the North Orbital Road. It was square shaped and was located around 100 metres to the south east of its current position. The M10 opened in 1959 and the roundabout moved slightly to make room for the fifth arm. It has remained the same ever since.
Big changes to the roundabout would have occurred if one of the alternative plans to complete the northwest section of the M25
was chosen. One of the options was to build an offline M25 between Hunton Bridge
and Bignells Corner
which follows the route as it does now and use the M10
for movements between the M1
east. This would involve grade separating all the intermediate junctions to enable traffic to flow unimpeded and Park Street would have had a simple M10 to A405 east flyover.
Online A405 upgrade with M10 merge
Another option was an online A405 upgrade west of Park Street with a new route paralleling the A405 between there and London Colney Roundabout
. M1 to M25 east movements would have used the M10 so that movement is also freeflow and the junction would have been on three levels. There is no access between the roundabout and the new route to the east so traffic would have to use the existing A405.
Offline upgrade with M10 merge west of Park Street
An alternative to the above would have been an offline upgrade paralleling the A405 both to the west and the east. This option could have been built as all-purpose or motorway and the M10 would have gained an intermediate junction. Between this junction and Park Street it would have been widened to D4 or D4M to cater for the additional traffic. Like the above design there is no access from the roundabout to the new route east and once again the existing A405 caters for this traffic. There is however a slip road westbound from the new route to the A405 which passes beneath the roundabout.
Routes shown in Highway Code
The sign in the Highway Code is an advance direction sign coming from the Hatfield direction. With the exception of Birmingham (which used to be what would have been signed there), all the places are fictional. Road numbers (except the M1) have been adjusted - the M10 becomes M15, the A405 becomes the A105, the A5183 becomes the A1183 and the M25 becomes M14.