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camera stations. It is a road junction where the main road, approaching
London from the south west, forks left deviating by about 40° while the
right fork carries straight over, The main road has four lanes to the
south west and three lanes to the north east with a joining 2-lane section,
which is about 400 feet long, The appearance of the scene to London-bound
traffic is shown in Fig, 9 where the possible distraction of advertisement
hoardings, the difficulties of reading road direction signs and markings and
of recognising the character of the junction from a distance are made clear,
In the period lst January, 1956 to 15th July, 1959, local police reports
show that 60 accidents have occurred on this site of which 42 took place at or
near the main fork, In eight of these drivers' indecision is believed to have
played an important part, another ten involved some indecision and/or lack
of care, Seven of these accidents have been studied in detail by the
Laboratory, These studies support the conclusion that the accident
potential of this road junction can be reduced, (It is understood that this
junction is to be redesigned and will include a roundabout, )
A comparison was made between this survey and a dyeline copy of a
conventional 1/480 scale survey (Fig, 10) made of the site in 1936 by the
county engineer, No material topographical change in the site appears to
have been made since then, Kerb line differences amounting to 2 per cent
were found. It must be noted that the whole plot was compiled from six
pairs of photographs using only minor control points and, although the errors
in the dyeline copy were unknown, these were probably about 1 per cent. The
2 per cent difference is not inconsistent with the possible accumulation of
error in such a compilation,
In Great Britain when law court proceedings follow a road accident the
police are often required to produce a true scale road plan. This is done
by a plan drawer', normally a constable engaged on general duties, who
indicates distances of critical importance,
Fig, ll is reproduced from a dyeline copy of such a plan of the
above-mentioned site and is included for comparison with the other surveys,
Even the most casual comparison of Figs, 8, 10 and 11 (taking Fig. 10 as
the standard) shows that the stereophotogrammetric equipment employed
yields maps whose precision exceeds that normally accepted in court cases.
The routine use of photogrammetric equipment by the police in
Britain, however, would almost certainly require a change in legislation,
The alternative, of up-holding photogrammetry as a valid science of
measurement in each law case, can be both time-consuming and extremely
expensive, (17)
Stereophotogrammetry has been used to estimate car damage. The
following example, while not exceptionally spectacular, adequately demon-
Strates some essential points, The two views shown in Fig. 12 were taken.
Plots, at 1/25 actual scale, were compiled at 10-cm height intervals; two
of these plots are shown in Fig, 13, The car outlines are dotted, the contour
lines are at 90 and 130 cm from the ground. Gaps in the contours are due to
lack of detail, In fact, extensive plotting was possible only because the car
Was covered with discrete rain drops. Highlights and other reflections from