"The Ashridge Interchange Movement ('AIM') is a non-party
organisation that exists to promote the
Costs and Benefits
Costs & Benefits
Cost-Benefit analysis can be quite complex, assigning monetary values to savings in time, and so on, and taking into account the net effects of taxes. For information, see the Department for Transport's WebTAG site.
The simplified version below concentrates on the obvious cash costs and the savings to individual motorists. If a rigorous cost-benefit analysis were performed on these proposals, then the ratio of benefits to costs is likely to be higher.
In the mid-1990's, it was estimated that a well-designed Ashridge Interchange itself would cost around £5 million to build. Today, the figure would be between £10 million and £15 million.
The Central Berkshire Transportation Study of 1996 costed a simple diamond junction with the A321 (which it admitted would be impractical to build because of DfT guidelines on distances between junctions) would cost around £2 million.
The tunnel is more difficult to cost. However, there have been projects of similar size and complexity on the Continent, and one in particular in Stuttgart in 2004 (see detailed costings, in German) was a twin track tram tunnel more than one kilometre long, and cost around £15 million. The single-carriageway link road tunnel will be wider but around half the length, so it is reasonable to estimate £10 - £15 million for this as well.
The Station Link Road would be a separate project, and it is expected that Network Rail will gain a major benefit from eliminating the Level Crossing; the additional costs of a railway bridge would therefore be offset by the contribution that Network Rail or Airtrack should make.
Thousand of commuters and regular users of the M4 and A329(M) to Reading or Bracknell will make direct savings. Using the recognised figure of 40 p per mile, it is estimated that annual savings on these alone would be over £5 million, as shown in this estimate:
Of course, a proportion of the non-commuters will be driving larger vehicles such as vans or lorries, but no allowance has been made for these additional cost savings.
Standard cost / benefit analyses also factor in the time savings. The motorists and commercial vehicle drivers listed above will all save up to 70 hours a year.
Other less tangible savings would accrue from an improvement in air quality, etc.
A Briefing Note for the 1996 Central Berkshire Transportation Study ('CBTS') assessed A.M. Peak-Hour flows (between 8 and 9 A.M. only), and based on these estimates, there would be annual savings of just under 1 million miles.
We would need to gross-up the un-published CBTS calculations for traffic flows outside this period as shown below:
- On these estimates, savings would be over 2 million miles, representing around £1 million in direct costs alone. However, there are doubts that the computer simulations used by the CBTS could adequately model 'dog-leg' traffic flows such as Wokingham-Winnersh-M4 or Wokingham-Coppid Beech-M4. It is felt that the simulations would tend to under-estimate the diversion of traffic if the Interchange had been built.
Cost / Benefit Ratio
Road schemes are justified on the benefit to cost ratio. It seems that on the figures based on the 2001 Census data, a good case can be made for the scheme to be partially funded by central Government. Even at £30 million capital cost, the compensating savings in direct costs and time would effectively 'pay back' the costs in fewer than 6 years.
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