Design considerations:
dimensions
6.4.5 Information on the design of horizontal
deflection features, including chicanes, is available
from experience in mainland Europe (e.g. Danish
Road Directorate, 1991; Herrstedt et al., 1993;
CROW, 1998). However, to ensure compatibility with
conditions in the UK, the DfT commissioned track
trials to ascertain suitable design criteria for horizontal
deflections, and in particular chicanes. The mainparameters affecting the mean speed of cars through
the chicane were stagger length, free view width,
lane width and path angle (TAL 09/94; Sayer & Parry,
1994). See Figure 6.5.
6.4.6 The results of the study are summarised
in Tables 6.1, 6.2 and 6.3. Chicane dimensions that
were suitable for reducing the mean speed of cars
to around 20 mph reduced the speed of buses and
coaches to about 10 mph, but were too tight to allow
an articulated lorry to pass.
6.4.7 Installing speed cushions on the approach to
a chicane would partially compensate for the longer
stagger lengths required to accommodate large
vehicles, while keeping the speed of cars to around 20
mph. An alternative approach is to use overrun areas
to give car drivers the impression of a restricted-width
carriageway, but allowing additional manoeuvring
room for large vehicles (see Section 5.5).