Chapter 3 – Part four: A central metropolitan block
Alternative arrangements of the grid: 305–309
What we wanted to discover was which of these layouts would permit the greatest amount of traffic to be generated by the area within the grid
Alternative arrangements of the grid
305We examined four possible rectangular arrangements for the grid (Figure 175) based on an absolute minimum distance between intersections of 1,500 ft. and maximum possible flows at the interchange ramps of 3,000 p.c.u. per hour (i.e. two-lane ramps running to capacity into additional lanes on the motorway). The smallest of these arrangements enclosed the study area tightly. What we wanted to discover was which of these layouts would permit the greatest amount of traffic to be generated by the area within the grid without overloading the system, with some idea of the actual quantity, so that we could proceed with the design of internal roads and parking spaces for the study area.
Assuming the evening peak hour to be the more concentrated of the two, it was a simple matter to calculate for each arrangement of the grid the rate at which departing traffic could leave the area having regard to the capacity of the ramps. The figures are given in line 4 of Table 9.
Table 9: Grid capacities and maximum permissible traffic generation for the four arrangements shown in Figure 175
| A | B | C | D | ||
|---|---|---|---|---|---|
| 1 | Grid dimensions (feet) | 3000 x 2150 | 3000 × 3000 | 4500 x 4500 | 6000 x 6000 |
| 2 | Area enclosed (acres) | 148 | 208 | 467 | 832 |
| 3 | No. of ramps | 2 | 4 | 8 | 12 |
| 4 | Possible traffic exodus rate p.c.u. /hr. | 6,000 | 12,000 | 24,000 | 36,000 |
| 5 | Additional internal generation p.c.u. hr. | 1,200 | 4,700 | 14,000 | 24,000 |
| 6 | Total permissible generation for the grid area p.c.u. /hr. | 7,200 | 16,700 | 38,000 | 60,000 |
| 7 | Study area as % of grid area | 100 | 71 | 32 | 17 |
| 8 | Total permissible generation for study area p.c.u./hr. | 7,200 | 11,850 | 12,200 | 10,200 |
We also had to estimate the additional internal generation, within the areas enclosed by the grid, of traffic which would not use the grid. This we were able to do with the help of information available regarding the proportions of London journeys in various categories of length (e.g. about one third of all car trips are less than a mile, slightly less than one fifth are between one and two miles, about one tenth are between two and three miles, and so on in diminishing proportions). These figures are shown in line 5 of Table 9. Line 6 of the Table shows the ‘total permissible generation’ for each grid area—if generation exceeds this figure, then congestion at the ramps will occur.
308As the smallest of the four grids is identical with the study area, the figure in line 6 is therefore the actual total permissible generation (line 8) for the study area. In the case of the other three grids the study area occupies only a part of the whole area enclosed, so the total permissible generation for the study area is estimated in proportion to the amount of the grid area occupied by the study area.
309Table 9 shows that the 4,500 ft. square grid would permit the highest level of generation per hour. Below this size a fall occurs for the reasons stated in paragraph 304, and successive increases in size also show a decrease of generation. In the studies that follow, the designs rest on the assumption that in each case the permissible level of generation is taken up to its fullest extent. Its allocation between essential and optional traffic is explained in a later paragraph.