|
Table
1
|
|
Stagger
length and car speeds
|
|
Lane
width 'B' (feet)
|
Free
view width 'A' (feet)
|
Stagger
length 'L' to achieve the required vehicle speed in chicane
|
| |
|
15mph
|
20mph
|
25mph
|
|
9.84’
|
+3.3’
|
19.7’
|
29.5’
|
45.9’
|
|
0
|
29.5’
|
42.7’
|
59.1’
|
|
-3.3’
|
39.4’
|
52.5’
|
-
|
|
11.50’
|
+3.3’
|
-
|
-
|
36.1’
|
|
0
|
29.5’
|
39.4’
|
49.2’
|
|
-3.3’
|
36.1’
|
49.2’
|
62.3’
|
|
13.1’
|
+3.3’
|
-
|
23.0’
|
29.5’
|
|
0
|
-
|
29.5’
|
39.4’
|
|
-3.3’
|
-
|
36.1’
|
49.2’
|
|
Table
2
|
|
Minimum
dimensions of stagger length for larger vehicles
|
|
|
Stagger
length 'L' (feet) needed for a free view width of 0.0 ft
|
|
Lane
width
|
9.8’
|
11.5’
|
13.1’
|
|
Artic.
Truck
|
65.6’
|
49.2’
|
36.1’
|
|
Rigid
Truck
|
39.4’
|
29.5’
|
23.0’
|
|
Bus
|
42.7’
|
36.1’
|
29.5’
|
NOTES:
Stagger
length (L)
The length between the start of the stagger on the offside and the
finish of the stagger on the nearside, or vice versa, for a one-way
approach. Increasing the stagger length increased mean speeds. Short
stagger lengths prevented the passage of large vehicles such as articulated
vehicles.
Lane width (B)
The width of the approach lane. There was little effect on speeds
when the approach lane widths were varied. The trials indicated that
drivers were prepared to remain behind cyclists where it was obvious
that there was insufficient space to pass.
Free view width (A)
The offset between the offside curb and the nearside curb as viewed
through the stagger, on a one-way approach. Changing the free view
width from +1m to -1m reduced mean speeds by about 10mph.
Visual Restriction
Obscuring forward visibility across the bulbouts. This had a positive
effect in reducing speeds by 2mph to 4mph. To avoid compromising the
safety of children, any barrier provided for this purpose should not
be higher than 600mm. Although in the trials the barrier was built
up to the carriageway edge, it will normally be preferable to provide
a clearance between the barrier and the carriageway edge to avoid
the barrier being struck. Reductions in speed may then be less.
