This is an average using TfL’s own screenline data for 2010, for the period between 7am and 10am, northbound.
Update: I realise this number is not that meaningful on its own. At the same point, there will be around 6 private cars, and 4 taxis. Spreadsheet with working here (cars and a qualifier in the second worksheet).
We know from Cyclists in the City that there are 1926 bicycles heading north over Blackfriars Bridge between 7 and 10am.
This means that on an average minute 11 bikes begin to cross the bridge. The length of the bridge, including the north junction (which I include because it’s what TfL are proposing to add an extra lane to) is 1300 feet, or 0.25 miles:
This means that, excluding traffic lights, it takes the average cyclist 1m15s to traverse the bridge.
There are two sets of lights at the north end of the bridge. There is effectively one phase for cyclists travelling north or west along the embankment, and one for those travelling east/north east (i.e. turning right).
This morning, I timed these phases. This allows us to calculate how many cyclists there are on the bridge, on average at any time, using the following variables:
a = cyclists who begin to cross the bridge per minute
b = length of bridge (miles)
c = average cyclist speed (miles per hour)
d = percentage of cyclists going straight on or left
e = percentage of cyclists turning right
f = length of green light phase for (d) in seconds
g = length of green light phase for (e) in seconds
h = length of red light phase for (d) in seconds
i = length of red light phase for (e) in seconds
j = average time/bridge during green phase
k = total traffic light phase time (ignoring amber)
y = adjusted average time
x = average number of cyclists on bridge at any time
You can see the values in this spreadsheet. We can plug them into this formula:
This tells us that the weighted average time for crossing the bridge is 95 seconds, and we can then calculate the average number of cyclists on the bridge at any time with the formula:
Many thanks to my friend Andrew for talking through the maths here with me when I thought my head was going to explode.
TfL are suggesting that at the north end of the bridge, the lanes for motor vehicles should be 10.1 metres wide, with the bike lane just 150cm. This is despite the fact that pedal cycles make up 36% of traffic over the bridge, with private cars at 19% and taxis at 12%.
I don’t want to jostle in a 150cm lane with more than a full rugby team’s worth of other cyclists while three wide lanes of traffic roar past. If you feel similarly, why not find your London Assembly member and tell them so?