Appendix 2: Cost-benefit analysis and accessibility and environment

Accessibility

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Accessibility can be described as the relationship between the capacity of an area to accommodate vehicles and the number of vehicles seeking to enter and stop within it. Thus in general, the accessibility is equal to the ratio between the ‘supply of road space’ and the ‘demand for road space’. Using Crude Capacity as a measure of the supply, and the potential (peak hour) generation as a measure of the demand,

Accessibility = Crude Capacity/Potential Generation

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Every developed area has a certain capacity to accommodate vehicles, and whether vehicles make use of it in small or large numbers, so long as the physical arrangements are fixed, so is the capacity fixed. In contrast, accessibility is a relative measure: an area laid out in a certain way in 1930 might have had good accessibility when car ownership was low, but as it or vehicle-usage increases, accessibility would fall, since such increases would imply that fewer of the potential vehicle trips or 'parking acts' could be made in a totally unhindered way.

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So far, accessibility has been described simply in terms of the capacity of an area and the demand for its use by vehicles. But the accessibility is also conditioned by the quality of the layout, and there are four aspects which need to be considered: the safety of the layout for vehicle-use; the distribution of the parking and loading facilities; the suitability of the internal routes to allow vehicles to move directly from one part of the area to another; and the convenience of the layout in other respects for vehicle-users.

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These four aspects are taken into account in determining the accessibility of the Newbury schemes. The schemes were tested against the following check list. Each aspect has been allotted a maximum possible score according to its possible influence (arbitrarily weighted) on accessibility. The total score (A) for any scheme cannot exceed 100.

Check-list for adequacy of the layout for vehicle-use (A)

Item	Aspects taken into account	Maximum possible total score
Safety	1. No conflicts between vehicles at links to network or on internal roads 2. No avoidable vehicular conflicts at stopping places or at places where motorists become pedestrians (e.g. at car parks)	20 15
		40
Distribution	1. Adequate distribution of the loading, waiting and servicing facilities for essential vehicles 2. Adequate distribution of the car parks	15 10
		25
Penetration and Directness	1. Adequacy of the road system to allow vehicles to penetrate close to build-ings; vehicles essential and optional 2. Adequacy of the road system to allow vehicles to move directly from one part of the area to another	12 8
		20
Convenience	1. Adequacy of layout for ease of move-ment, turning and manoeuvring for essential and optional vehicles (in access roads and from car parks, etc.) 2. Clarity for motorist of the road lay-out; interest from motorists' eyeview	10 5
		15
	Maximum possible total score	100

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On the basis of the foregoing, we have constructed a measure of accessibility:

Accessibility index = Crude Capacity x A / Potential generation

All the Newbury schemes would be subject to the estimated potential peak hour generation (for the year 2010), of 3,000 v.p.h. For purposes of comparison, the present potential generation has been taken as 1,250 v.p.h. Table 3 below shows the (A) scores and the Accessibility Indices of the present layout of Newbury, and Schemes A, B, and C.

Table 3: The accessibility of the Newbury schemes

	Present conditions	With year 2010 potential generation
		Present layout	Scheme A	Scheme B	Scheme C
Crude Capacity v.p.h.	1,600	1,600	1,700	3,500	3,500
(A) score	54	54	60	68	78
Potential generation (v.p.h.)	1,250	3,000	3,000	3,000	3,000
Accessibility Index	69	29	34	79	91

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The Table shows that if no changes are made to the present layout, accessibility would fall by the year 2010, to less than half its present value, solely as a result of the additional numbers of potential users. Schemes A, B, and C would result in progressive improvements resulting from increases in Crude Capacity and also in the values of (A). By the year 2010, the accessibility of the restricted network scheme (Scheme A) would, in spite of the construction of the new network, fall to only half its present level, but Scheme C would have an Accessibility Index about 1.3 times the figure for present conditions.