A48 Capacity

Summary
  • The LDP gives the A48 capacity as a WS2 type of road.
  • St Nicholas village should be designated as a UAP3 type of road - thus with less capacity.
  • Lower speeds increase capacity - junctions and possible obstructions (parked cars, road works, etc.) limit capacity
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The LDP has tables concerning the capacity of road links (links run between junctions)  For St Nicholas, the link capacity used is outside of the village, where the speed limit is 60 mph.  This link is numbered 196 in the LDP.

Using Department for transport (DfT) documents, we can say that in St Nicholas, the road is an Urban All-purpose (UAP3), and at about 7.7m in width outside the former post office has a maximum capacity of about 1300 vehicles in any one hour period - see DfT tables below.

DfT Tables:

Road Type:


Capacity:

TA 79/99 Advice Note gives the maximum hourly vehicle capacity for various types of Urban Trunk Road - 15% HGV.


Narrower roads are given a lower capacity due to the increased potential for significant obstructions such as parked cars and road works.

"The capacity of the lower width roads will be significantly reduced by parking and temporary width restrictions caused by such activities as maintenance and Statutory Undertakers' Works. The lowest widths are unlikely to be suitable for bus routes or for significant volumes of heavy goods vehicles."

Whilst the A48 outside the village has been given a capacity of 1860, and new developments along the A48 are expected to use the excess capacity this figure gives, similar roads in the Vale of Glamorgan, are given more appropriate capacities.

How the A48 compares to other links in the Vale of Glamorgan:

Link no. LocationSpeed limit Link Type Max. capacity
 196 A48 (Nr. St Nicholas)60 Rural S10(Good) 1860
 192 A422 Aberthin Road30  B road single carriageway  1270
 204 Pontypridd Road, Barry 30  Small town 90% devel  880
208  Redlands Road, Penarth 30 B road single carriageway 1270 
209  Cardiff Road, Dinas Powys (north)30 Small town 35% devel 1300 
222  Colcot Road, Barry 30  A road single carriageway 1270 
 223 Port Road West (Tesco) 40 A road single carriageway 1270 
 224 Port Road (Merthyr Dyfan Road) 40  A road single carriageway  1270
 189 A48 (East of Pentre Meyrick)30 Small town 35% devel  1300 
all figures from the LDP deposit - including 30 mph on the A48 at Pentre Meyrick


This is further supported by the report from the public inquiry into (Trunking) Order 200 of the A48 which states:  

5.41 AG’s contention that there is spare capacity on the A48 is not accepted. At busy times it is extremely difficult to access the A48 east-bound from the side road from the Downs, as traffic is stationery or moving nose to tail. On some occasions during the a.m. peak, these tail backs extend from Culverhouse Cross back as far as Bonvilston to the west of Sycamore Cross on the A48. When gaps in traffic do occur, entry into the east-bound lane is frustrated by west-bound traffic (ID/O10/1, page 1).



The capacity of the A48 through St Nicholas can be no more than 1300 vehicles in any one hour.


The LDP gives St Nicholas as having a peak time flow of 1129 eastbound in 2012, rising to 1277 in 2026.

Figures from September 2013 show an averaged average of 1180 cars eastbound in one hour in the morning peak.

1230 vehicles traveled eastbound through St Nicholas between 7:45 and 8:45 on Thursday September 19, 2013.

The council states that capacity values of 90% or less are typically regarded as being within capacity. 

If we take 1300 to be the capacity of the A48 through St Nicholas, and the councils 2012 figure of 1129 vehicles and 2026 figure of 1277, then in 2012, capacity was at 87% and in 2016, it will be at 98%
 
The given capacity for Port Road in Barry is 1275, which would put st. Nicholas at 89% in 2012, 1230


Ireland's Design Manual for Urban Roads and Streets (based on the UK's Manual for Streets states:  Slower vehicle speeds are often perceived to be a cause of congestion but can lead to increased traffic capacity.

A reduction of the speed limit through the heart of St Nicholas would therefore not reduce the roads capacity - with many vehicles in the AM peak hour not reaching speeds in parts of St Nicholas above 20 mph today.







In 2006, the Welsh Government consulted on the possibility of a bypass for St Nicholas: