[sustran] motorcycles and congestion

[Kerry Wood]

Hi everybody


Problems of bicycle capacity: The answer seems to be very messy,
particularly if the available width is less than about 2 m.

The CROW manual ( Netherlands, 1993) gives capacities of 3300 cycles/h for
a streaming lane 1.0 m wide and 4700 /h for 1.8 m wide. Minimum widths are
given as 1.5 m for cycling 2 abreast, 2.5 m for cycling 3 abreast etc. The
capacity/width curve is in fact a 'fuzzy' step function: two cyclists
cannot ride side by side in 1.0 m width, can just ride side by side in 1.5
m (probably with some delay) and can ride without delay in 1.8 m. But by
then cyclists can ride 3 wide with some delay... The curve can be
conservatively plotted as tangential to the steps

        Capacity = [(Width - 0.8 m) x 3300] +1000       cycles/hr

This is clearly conservative in some cases: Homburger et al (1996) give
capacity as up to 2600 cycles/hr.m width, and describe 1300-1960 cycles/
hr.m as level of service 'C'. However, they also give the speed as 15-17
km/h (and down to 10-13 km/h at capacity), so perhaps some conservatism is
appropriate.

Making some assumptions about traffic capacity, and assuming 1.44 persons
per motor vehicle (average implied in a New Zealand household travel
survey)

Available width (m) --------    Capacity (persons/hr)   Capacity
Total   Traffic         Cycles  Traffic Cycles  Total   increase with
                                                        cycle facility

3.0     3.0             -       2400    -        2400
        -               3.0*    -       8200     8200    3.4 x

3.5     3.5             -       2400    -        2400
        -               3.5*    -       9900     9900    4.1 x

4.0     4.0             -       2500    -        2500
        -               4.0*    -       11500    11500  4.6 x

4.5     4.5             -       2600    -        2600
        3.0             1.5     2400    3300     5300   2.0 x

5.0     5.0             -       2600    -        2600
        3.5             1.5     2400    3300     5700   2.2 x
        3.0             1.5**   2400    3300     5700   2.2 x
        3.0             2.0     2400    5000     7000   2.7 x

5.5     5.5             -       2600    -        2600
        3.5             1.5**   2400    3300     5700   2.2 x
        3.0             2.0**   2400    5000     7000   2.7 x
        3.0             2.5     2400    6600     8600   3.3 x

7.0     2 x 3.5         -       4800    -        4800
        3.0 + 2.5       1.5     4600    3300     7900   1.6 x

8.0     2 x 4.0         -       5000    -        5000
8.0     2 x 3.0         2.0     4800    5000     9800   2.0 x
8.0     2 x 3.25        1.5     4800    3300     8100   1.6 x
8.0     2 x 3.0         1.5**   4800    3300     8100   1.6 x

*  Not recommended without physical separation (too easy to use as a car lane)
** Additional width used to separate cycle and motor traffic


If the table falls to pieces in transmission, the full headings are

        Total available width (m)
        Traffic available width (m)
        Cycles avaialble width (m)
        Traffic capacity (persons/hr, assuming 1.44 per car)
        Cycle capacity (persosns/hr using the equation above and assuming 1
person per bike)
        Capacity increase (cycle capacity / car capacity)


References are:

CROW (1993) Sign up for the bike, design manual for a cycle-friendly
infrastructure. Netherlands: Centre for Research and Contract
Standardisation in Civil Engineering
Homburger, WS, et al (1996)     Fundamentals of Traffic Engineering.
Berkeley, California: Institute of Transportation Studies.





Picking up on Todd Litman's comment:

>Bicycles are sometimes considered a problem if they increase traffic delays
>and accident risk. For analysis of bicycle traffic impacts, road conditions
>are divided into four classes:
>....

Bikes are often faster than cars in congested conditions, which makes it
hard to argue that they are significantly delaying the cars.





New but vaguely related subject:

Roelof Wittink points out that in the Netherlands bikes are safer than cars
for high-risk road users, and I have found the same for 15-20 year olds in
New Zealand: About twice as safe for overall crash costs, but I have not
broken it down by safety-to-self and safety-to-others.



Regards




Kerry Wood
Transport Consultant
Phone/fax + 64 4 801 5549  e-mail kwood at central.co.nz
1 McFarlane St  Wellington 6001  New Zealand