M50 Motorway (Ireland) - Layout

Layout

The M50 was originally planned to divert traffic traveling on National Primary Routes away from the city (a full bypass of Dublin). Due to urban expansion it now runs through Dublin's suburbs and serves a route for Dublin itself, connecting the suburbs.

All of the National Primary Routes leaving Dublin have junctions with the M50. The intersections were originally in the form of grade-separated signal-controlled roundabout junctions, not free-flowing interchanges. The M50 mainline itself was free-flow through all junctions. The other primary routes served are the N2 to Derry/Monaghan, N3 to Navan/Cavan/Northwest, N4/M4 to Galway/Sligo, N7/M7 (M8) to Cork/Limerick/Waterford, and the N11/M11 to Wexford. Additional junctions along the motorway serve other suburbs of Dublin such as Ballymun, Tallaght, Naas, Dundrum, Sandyford and Cherrywood.

Most of these interchanges were subject to high levels of traffic congestion, as was the former toll-plaza north of the West-Link bridge. The busier roundabout junctions were signal-controlled, with tailbacks extending for several kilometres at rush hour. The most infamous was the Red Cow roundabout junction with the N7, formerly dubbed the "Mad Cow Roundabout". As well as being the junction of two of the busiest roads in the State, the Luas tram Red Line from Tallaght to the city centre used to cross two slip roads at-grade, before continuing city-bound in the median of the R110 (formerly N7). As part of the M50 upgrade works (see below), these at-grade crossings were removed, and on December 2008, the completely reconstructed interchange was open. This greatly reduced the congestion at the once-notorious traffic blackspot.

The roundabout at the N3 is also notable as the Royal Canal and the Dublin-Sligo railway line pass through its centre.

The original speed limit on the M50 was 70 mph (112 km/h) which was increased to 120 km/h when all speed limits in the Republic of Ireland became metric in 2005. The Southern Cross Route (the extension from Junction 12 to Junction 14) was given a lower limit of 60 mph (96 km/h) with a decimal limit of 100 km/h after 2005. This was because of its more undulating terrain, tighter and more frequent curves and resulting shorter sight lines. Because of the doubling of the number of running lanes along most of the route, which has shortened sight lines and increased lane changing, and due of the high number of junctions, the entire length of the route has (with the exception of the sections from Junction 1 to Junction 3 and Junction 14 to 17) the same speed limit, of 100 km/h, since it was completely upgraded (2007–10).

The section of the route which was previously the M1 from Junction 1 to Junction 3 (which is now the M50 up to the port tunnel) retains a speed limit of 80 km/h due to the closely packed junctions, and because of the high number and frequency of lane changes that are required among all manner of vehicles depending on their intended route. For example, traffic traveling Southbound on the M50 approaching the Dublin Port Tunnel must cross 2-3 lanes of traffic to avoid its portals. Whilst in the opposite direction, traffic exiting the tunnel must cross 3 lanes to continue on the M50 rather than progress onto the M1.