The R148 road is one of Ireland's regional roads which was classified following the opening of a bypassed national primary road. The section of the old N4 which has been by-passed by the M4 motorway was reclassified R148. The R148 runs from Dublin to Kinnegad in County Westmeath. The road is 45 km long. It is generally of a very high standard for a regional road, with wide lanes, hard shoulders, and turning bays. It is still heavily used by traffic avoiding the tolled M4 between Kilcock and Kinnegad. The road was previously categorised as a National Primary route until the opening of the M4 motorway and therefore carried a speed limit of 100 km/h. The speed limit has now been reduced to 80 km/h. The fact that speed limits have been reduced, even though traffic is lighter than before, has led some to claim that the limit reduction is simply a cynical attempt to force motorists to use the tolled M4. This argument is perhaps strengthened by the fact that the R148 received a large share of the speed cameras that were rolled out across Irish roads in 2008. However, under the Road Traffic Act 2004, the default speed limit on Irish regional roads is 80 km/h. A section of the road (between Kilcock and Enfield had for safety reasons already been subject to a special speed limit of 50 mph/80 km/h for some years prior to M4's opening in any case.

Route

The route starts (east-west) at Aston Quay in central Dublin. It then runs to Palmerstown where it becomes the N4. It starts again in Lucan, County Dublin at the same grade-separated interchange at which the M4 begins. Less than 1 km west it crosses the River Liffey into Leixlip in County Kildare. It then passes through Leixlip, Maynooth, Kilcock, Enfield, and Clonard before rejoining the N4 west of Kinnegad. The official description of the R148 from the Roads Act 1993 (Classification of Regional Roads) Order 2012 reads: