It is often argued that a new major new offline bypass at Arundel would prevent rat-running through the South Downs National Park, and thus for example improve air quality at Storrington.
This argument is used to justify plans for severe damage to the South Downs National Park and its setting from major new road-building. But the idea of a net, or even any significant beneficial impact, is unconvincing. Here are some of the reasons why this ‘net benefit to the National Park’ argument, however superficially plausible, is wrong.
- Much or most traffic on NP roads would continue to use existing routes.
There is no evidence that most drivers wouldn’t continue to use their existing preferred routes in or through the National Park. Many would just follow their SatNavs as to the shortest routes from A to B – which may still be the same as now. Any drivers who actually are going via Storrington to avoid the slower Arundel/Worthing A27 sections would not change that habit unless there were a high speed route through Worthing as well as Arundel.
There will not be such a high speed route, because traffic flow improvements are proposed at Worthing but not a high speed offline bypass. This was rejected because of the negative impacts on the National Park. Pauses in the flow of traffic at Worthing will continue to be essential for local access onto and across the A27. The design speed at Worthing will be max 40-50mph.
So, any Storrington traffic which might currently be going there to avoid Worthing, will still do so, even if a high speed Arundel bypass is built through or close to the National Park.
- Induced traffic is not taken into account in the current plans.
Highways England has recently confirmed that induced traffic was not taken into account in the A27 Feasibility Study reports or in the cost/benefit assessments of the plans. The effects of induced traffic would be to increase traffic more than expected in the whole area (including in the South Downs, as more people start and end those ‘suppressed trips’ locally). Greater increases in traffic than forecast would lead to traffic jams and these would lead to continuation of traffic jam avoidance by journeys through the South Downs National Park.
This admission casts serious doubt on the ‘Storrington argument’ for an Arundel Bypass, and also on the cost-benefit analyses provided by the reports.
The 1994 SACTRA report (the Standing Advisory Committee on Trunk Road Assessment’s report ‘Trunk roads and the generation of traffic’) proved the reality of ‘induced traffic’ from road improvements. It said (p. iii) that induced traffic is of greatest importance in the following circumstances: ‘where the network is operating or is expected to operate close to capacity’ and ‘where traveller responsiveness to changes in travel times or costs is high, as may occur where trips are suppressed by congestion and then released when the network is improved’.
The A27 Feasibility Study’s Report 1, March 2015, 4.7.2, shows that the A27 at Arundel and Worthing is operating above capacity. Therefore at Arundel, and even more at Worthing, a significant amount of induced traffic could be expected from upgrading the single carriageway sections to high speed dual carriageway.
- Building new roads causes more traffic congestion – including in the National Park.
Even if induced traffic were allowed for, all new roads block up eventually, i.e. demand increases until the new road is full. This has been shown by the study ‘The Fundamental Law of Road Congestion: evidence from US cities’, by Giles Duranton and Matthew A. Turner of the University of Toronto and London School of Economics, published in 2011 in the ‘American Economic Review’. The Study’s findings include
- The number of VKT (vehicle kilometres travelled) increases in direct proportion to the available lane-kilometres of roadways. The additional VKT travelled come from increased driving by current residents and businesses, and migration.
- Building new roads and widening existing ones only results in additional traffic that continues to rise until congestion returns to the previous level. Such attempts to ‘cure’ congestion are thus both expensive and ineffective.
- Increasing lane kilometres for one type of road does not significantly reduce congestion on others – for example, widening highways does little to reduce local congestion.
This summary suggests that any ‘relief’ to roads in the Downs, including at Storrington, given by new road-building on the A27 will be insignificant, and even if it does take place, will be temporary.
- Once congestion has ‘returned to the previous level’, rat-running would again take place.
- A busier A27 would cause concentrations of local traffic trying to access or cross it.
- A busier A27 would bring more visitors to the Park by car rather than other modes.
Temporary relief is not a good enough reason to cause damage to the South Downs National Park and the beautiful areas that surround it by new roadbuilding.
If temporary relief is possible, the old ‘Purple’ route would do it better, because it would not induce so much extra traffic. This is because it would be designed for max 40-50mph.
It would also do this without causing major new damage to the Arundel watermeadows, beautiful countryside and ancient woodland – genuinely benefiting the Park and its setting.
Emma Tristram 28 October 2015
 See A27 Feasibility Study report 1, March 2015, 4.4.2, ‘There is currently an AQMA at Storrington along the A283, which is affected by traffic re-routing off the A27 in order to bypass Arundel and Worthing’.
 Highways England’s response to the FOI request by SCATE, the South Coast Alliance for Transport and the Environment, dated 13 October 2015, confirms that ‘No induced traffic was assumed. A fixed matrix highway assignment model was used for the assessment as indicated in Section 4.3 of report 3 of 3 Investment Cases’ (http://scate.org.uk/wp-content/uploads/2015/10/Mr-Chris-Todd-FOI-2nd-request.pdf) .
 ‘The average traffic volumes are close to or above the theoretical capacity of the road (a single carriageway road is expected to accommodate AADT volumes of up to 13,000 vehicles) along the single carriageway sections, for instance AADT volumes in 2013 were over 15,300 at Arundel, over 17,800 at Worthing and over 11,400 between Lewes and Polegate’.