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B.
Detailed comments on the route through Ealing
C. Appendix
i)
Lane Width
ii)
Permeability Survey
C.
Appendix
i)
Evidence against 4m-wide traffic lanes for all vehicles including
buses, HGVs and cycles. Colin McKenzie 28
7 2003.
The West London tram currently proposes one-way general traffic
lanes 4m wide, bounded by a kerb on one side and the tram reservation
on the other. This is supposed to be wide enough for motor vehicles
to overtake cyclists. In my view, based on a bus or HGV width of
2.55m, the necessary width is 4.35 – 4.5m.
Evidence:
1. Wilkinson et al 1994 (Wilkinson, Clarke, Epperson
and Knoblauch Selecting Roadway Design Treatments to Accommodate
Bicycles) FHWA-RD-92-073
This is a very important document because Cycle-friendly Infrastructure
and many cycle campaigners use it as the basis for their views on
carriageway widths. It is based on studies of real cyclists and
drivers in the USA.
The document suggests 4.2m as the minimum width for a ‘wide
curb lane’ – wide enough for motor vehicles to overtake
cyclists without encroaching on the next lane.But (on page 22) it
says (my emphasis):
‘Most practitioners agree that 14’ (4.2m) - usually
measured from the lane stripe to the edge of the gutter pan, rather
than the curb face - is the minimum width necessary to
allow a bicyclist and motorist to share the same lane without coming
into conflict, changing lanes, or potentially reducing the motor
vehicle capacity of the lane. Where traffic speeds exceed 40 mph,
and where annual average daily traffic exceeds 10,000, 15- or 16-foot
(4.5- or 4.8-m) lanes are considered desirable.’
In my view the significance of the section in bold has been missed
by all who have relied on this document. On page 23 the same report
says cycle lanes should be 1.5m wide, or at least 1.2m measured
from the outer edge of the gutter pan.
This implies a typical ‘gutter pan’ width of 0.3m, and
a total width for a wide kerb lane (kerb to white line) of 4.5m.
In Britain drains project at least as far from the kerb.
This means FHWA-RD-92-073 is actually recommending about 4.5m from
kerb to white line – just as I am. 4.5m is still just acceptable
if the outer white line is replaced by a kerb or tram reservation.
2. Calculation
of relevant widths and clearances
a. Bus and HGV width.
Current limit 2.55m. When Cycle-friendly Infrastructure was written,
the limit was 2.50m.
b. Distance of cyclist from kerb.
Camden Cycling Campaign recommend cyclists to ride 1m from the kerb
- absolute minimum 0.6m. Any less and you are liable to hit pedestrians’
bags and elbows, or put the wheel down a drain. This distance refers
to the bike wheels.
c. Width
of cyclist
0.5 to 0.6m (handlebars and shoulders). But tricycles and trailers
(including child trailers) are wider – up to 1m. Half of this
is already counted as part of the distance from the kerb to where
the wheels run.
d. Safe overtaking clearance
If the overtaking vehicle is within reach of the cyclist's outstretched
arm (about 0.8m from the shoulder), it is too close, at any speed.
This is in line with the often-quoted minimum clearance of 1 metre.
This gives total minimum widths:
Kerb to inside of bus etc: 1.85m
Kerb to outside of same: 4.40m.
The actual lane width required depends what is on the offside –
kerb, tram reservation, another lane, width and direction of next
lane.
3. Traffic
Advisory leaflet 01/97 Cyclists at road narrowings
It says:
“Local Transport Note 2/95 recommends that where a pedestrian
refuge island is introduced, a vehicle lane width of 4.5m should
be maintained. Whilst this allows motor vehicles to pass cyclists
safely, it has little or no speed controlling effect.
“The extent to which motorists will overtake cyclists within
a narrowing will vary depending on the characteristics of the site.
In the schemes studied, this was less sensitive to running lane
widths than to other site-specific characteristics. It should normally
be anticipated that at least 70% of drivers will attempt to overtake
a cyclist within or close to a 3.5m narrowing.
The interview surveys showed that the proximity of motor vehicles
at a narrowing was of concern to cyclists, especially at the point
where the carriageway begins to be narrowed.”
4. ‘Cycle-friendly Infrastructure’ DfT/CTC, 1996
(P42) Suggests that 4.25m is just OK for a bus lane - but this is
with 2.50m wide buses.
(p43; section 11.1.2) says the same for ‘wide nearside lanes’,
following Wilkinson et al 1994:
“This is a crucial dimension: if narrower, cyclists will not
have sufficient clearance from passing HGVs; if wider, traffic will
tend to double up and create two sub-standard lanes.”
Note that this is for the situation where there is another lane
in the same direction on the offside.
(p43, section 11.2.2) on with-flow bus/cycle lanes says:
“Three metres is the minimum width for a bus lane. However,
this does not permit a bus (2.5m maximum width) to overtake a cyclist
without leaving the lane. Where the carriageway is wide enough,
a 4.25-4.6m lane should be provided.”
[Note the maximum bus width quoted - the limit is now 2.55m, so
increase both widths by 0.05m.)
Note also that a bus in a bus lane can encroach slightly on the
adjacent lane if necessary. This is probably why this publication
allows as little as 4m for contraflow bus lanes, though it prefers
4.25m. Again, add 0.05m.
5. Draft LCN+ design standards - courtesy John Lee, Cycling
Centre of Excellence
"Design Standards for the LCN+ are in draft form and will be
confirmed within the next few weeks. They
include a number of issues that are particularly relevant, primarily
the following key points:
? Minimum cycle lane (on carriageway) width of 1.5m
? Minimum cycle track or shared path normally 2.0m minimum
? 20mph speed limits where segregation (by lanes or tracks) cannot
be achieved, while ensuring clear routes for cyclists are provided
through congested areas
? Bus lane widths to be 4.5m incorporating 1.5m wide cycle lanes,
where there are moderate bus flows."
6. Empirical - what is frightening
If the overtaking vehicle is within reach of the cyclist's outstretched
arm (about 0.8m from the shoulder), it is too close, at any speed.
The required distance increases with speed. That gives a kerb to
inside of overtaking vehicle distance of 1.65m as definitely too
little. 1.85m is acceptable - possibly 1.8m if the speed limit is
30 or less and well enforced. Thus the absolute minimum kerb to
outside edge of bus or HGV is 4.35m. This is rounded up to 4.5m
as the bus cannot drive right on the edge of the lane if there's
a kerb there too. It probably can drive right on the edge of the
tram reservation, so 4.35m is the absolute minimum for the general
traffic lane if the aim is for it to be safe for any vehicle to
overtake a cyclist.
How
the plans could be modified
In many sections it is going to be difficult to find an extra 0.35m
width each way for the general traffic lanes. In part this is because
of the pressure to keep the Uxbridge Road open to general traffic
wherever possible.
Some ideas:
• Narrower trams. 2.4m wide trams (instead of 2.67) would
immediately solve the problem in most places, at the cost of a major
reduction in capacity. But standard bus width - 2.55m - would be
a sensible compromise.
• For short sections, a narrower general traffic lane one
way, with the full 4.4m the other. A 3.0m lane is wide enough for
a bus, but prevents most drivers from attempting to pass cycles.
This is a particularly good option on hills (e.g. Hanwell, Acton).
• Put cycles with the trams rather than the general traffic
- particularly just after stations where they have a chance to get
well ahead of the trams. Crossing the rails would have to be carefully
managed.
• Close more sections to general traffic!
• Width restrictions. Put the buses on the tram tracks and
limit other vehicles to 2.1m. These would only need a 3.95m wide
general lane to overtake cycles safely.
• Speed restrictions. Enforce a 20 mph limit and the need
to overtake cycles vanishes - in theory!
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