Chapter III – part two: A large town

Industrial, commercial and business journeys

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These journeys were by far the most difficult to analyse because of their great variety, and the dearth of information about the generation of this class of traffic. Fortunately, surveys had recently been made by the local authority of traffic crossing the city boundary on the main radials, of distribution depots within the city, and of all firms employing more than 5o persons. These provided us with some working material, and enabled us to distinguish between traffic associated with manufacturing and non-manufacturing industry, and gave a cross-section of the number of persons working in each type of industry in each postal district of the city, together with the total floor space occupied by each firm. From this we were able to evolve an average ratio of floor space per employee for each of 25 different types of industry (15 manufacturing and 1 non-manufacturing). We also obtained some additional information from a number of concerns by direct interview. These included British Road Services for their parcels and bulk transport, British Railways for their parcels, bulk transport and fish, meat, fruit and vegetable traffic, and Tetley‘s Brewery (Figure 114).

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From all this information we worked out present day ‘traffic generation factors’ for each of the 25 industrial groups. These factors were expressed in terms both of journeys per day per unit of floor space, or journeys per day per number of employees,† and they distinguished between journeys made by car, van or lorry. For the city as a whole we estimated the proportion of journeys made by each of these three types of vehicle, the results being shown in Table 2.

Table 2: Journeys made by cars, vans and lorries as a proportion of all journeys connected with industry

	Manufacturing Industries	Non-Manufacturing Industries	All industries
Cars	50%	32%	36%
Vans	10%	40%	31%
Lorries	40%	28%	33%
	100%	100%	100%

† For example, for manufacturing industries, generation factors ranged from 1 trip per 1,500 sq. ft. to 1 trip per 6,000 sq. ft. of floor area, and for non-manufacturing industries the range was from from 1 trip per 200 sq. ft. to 1 trip per 2,500 sq. ft.

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We then applied the traffic generation factors to a sample number of industrial firms in each of the employment zones (i.e. the zones which we had previously distinguished in connection with the analysis of journey to work), and by ‘scaling up’ we arrived at the total number of journeys for each zone.

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Thus far we had dealt only with traffic connected with manufacturing and non-manufacturing industry. The big remaining gap was the traffic connected with the distributive and service trades which account for 14% of the total labour force of Leeds. Traffic generation factors were again derived, based on the ratio of numbers of employees to floor space occupied. This procedure was carried out for the central area, and for each of the seven suburban shopping centres.

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Having arrived at an estimate of the number of vehicle movements generated by each employment zone, we then had to make broad assumptions as to the directions and length of the movements so generated. These assumptions, which are set out below were necessarily very tentative because there was little or no knowledge available.

Manufacturing industrial traffic	Percentage of total movements
Movements between industrial zones within the city and places outside the city	80%
Vans	20%

Non-manufacturing industrial traffic	Percentage of total movements
Movements between zones within the city and places outside	30%
Movements between zones within the city	25%
Movements between industrial zones and residential	20%
Movements taking place within the industrial zone of originy	25%
	100%

Distribution and service traffic	Percentage of total movements
Movements between zones	60%
Movements within the zone of origin	40%
	100%

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In estimating the directions of flow between zones within the city and places outside, we applied ‘weightings’ based on present-day volumes of traffic (as known in detail) on each of the radial roads. Flows between zones within the city were weighted in accordance with the amount of floor space in each zone. It was possible to check the results of our allocations as far as movements across the city boundary were concerned, against the known flows revealed by the local authority’s survey. They were found to give quite comparable results.

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Finally, since all these computations gave us only present day movements, we projected them to the year 2010 by means of a growth factor of 2.66*. Thus we were able to draw the third desire line diagram (Figure 115). This shows the movements in the year 2010 which would take place entirely within the city, and those which have an origin or destination outside the city.

* This projection, involving the use of a single global growth factor, is a very crude one, since it takes no account of the many modifications that will inevitably occur in the local pattern of industry, or in the manufacturing processes or the supply and delivery arrangements. It is, however, the only one possible until further studies are carried out in this field. It is related to the expected general increase in the total number of commercial vehicles in use by the year 2010 (3⅔ times the present day), but as this total fleet will be owned by, and operate from, many more business concerns than exist today, the growth factor assumed for the generation of journeys by a given amount of floor space is expected to be less than this increase, notwithstanding an expected slight increase in the use of each individual vehicle.