Chapter 5: General conclusions

Factors affecting the amount of optional peak period traffic

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Our studies showed that the amount of optional traffic that could be accommodated was, proportionately, greatest in the smallest town studied, while it was least for the London example. The Leeds study fell in between the two. The explanation for this apparently inverse relationship, is not, however, to be found strictly in terms of town size, so much as in three separate factors. These are the capacity of the primary distributory system, the density of development, and the duration of the peak period. This suggests that if, in any particular case, the proportion of optional traffic which can be accommodated is thought to be too low, there are three ways in which it could be increased:

1. by improving the capacity of the distributory system, including balanced parking allocations,
2. by reducing the potential number of optional users through reduction of density (i.e. removing some of the activities right out of the area so that fewer people work within it), and
3. by extending the length of time of the peak period.

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Method (i) above is likely to involve formidable constructional complications. Our studies show clearly the great scale upon which networks are required even to deal with quite moderate proportions of optional traffic. Method (ii) has to be undertaken on quite drastic a scale to achieve a big change in the proportion of optional traffic that could be accommodated. For example, in the Tottenham Court Road ‘complete redevelopment’ scheme, about two-thirds of the floor space would have to be put elsewhere to allow 100% of the optional traffic to enter the area. The floor area per acre of site would have to fall from 2.8 acres to about 1.0 acres. In the case of the Leeds study about half the floor area would have to be overspilled.

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Method (iii) may be said to be equally unrewarding. In our Leeds study, for example, we assumed that the evening peak period would have a duration of about one and three-quarter hours. The average flow during the peak hour, the basis for design, is about 25% above the average for the peak period. On the network, the peak-hour flow would generally be about twice the average hourly flow from 6 a.m. to 10 p.m. Even if some way could be found to persuade people to stagger their times of leaving work-places over a period of, say, 2½ hours, the proportion of the potential work-journeys by car that could be accommodated on the limited network would rise from 40% to only 55%. Employing the same principle, to accommodate the whole of the potential optional traffic, would require the times of leaving to be staggered over a total period of some 4½ hours.