Email from Klaus Grimar explaining the background of the cycle counts in Copenhagen as reported on my blog. Some confusion had been caused by people exaggerating the true counts.
from: Klaus Grimar <______@tmf.kk.dk>
to: David Hembrow
date: 25 January 2011 09:43
subject: bicycle counter
Dear David Hembrow
Thank you for your interest in the bicycle project on Norrebrogade in Copenhagen. I will be happy to explain how the bicycle counter works and how the number of cyclists measured daily by the counter compares to the officially published number of 37.000 cyclists in total per day on the street.
The most important reason for the difference is that the bicycle counter measures only the number of cyclists in the outgoing direction from the city centre. The counted number has therefore to be at least doubled in order to account for the total amount of cyclist on the street daily in both directions. That said we know that there is a significant difference in the number of cyclists travelling to and from the city during the day.
We know that during the morning rush hour people commute via the most direct route from their home to their place of work or study in the city centre using the bridge in the inbound direction. During the afternoon rush hour the traffic is most dense in the outbound direction where the automatic counter is measuring. We know that the bicycle traffic on the street in the afternoon rush hour is less than in the morning rush hour, and the reason for that seems to be that people tend to have leisure activities in different parts of the city during the afternoon and therefore many people tend not to travel via the same route back to their homes. The conclusion is therefore that there is more traffic in the inbound direction during the day than in the outbound direction, and the daily total number of cyclists on the street is therefore closer to around 28.000 cyclists on average in both directions on a normal working day throughout the year.
This number is still quite a bit less than the officially published number of 37.000 cyclists and the reasons for that can be explained by the following. Every year the Department of Traffic undertakes manual traffic counting surveys on the major streets in the city. Norrebrogade has received special attention in these surveys during the last couple of years due to the major changes the street has gone through during the project. The official number of 37.000 cyclists in total on the street daily on a working day comes from these surveys. The surveys are only made on single days and their results are therefore naturally dependent upon the weather and the time of year. The way the surveys are conducted is such that the traffic is only counted during 12 hours of the day from 7AM to 7PM on a normal working day of the week, defined as the days from Monday to Thursday. To account for the remaining hours of the day the numbers of vehicles and cyclists counted have to be corrected with a factor to adjust them to represent the traffic over the entire 24 hour period. This factor has been set to 1.22 based upon experience from numerous traffic surveys in the city in previous years.
The survey that resulted in the number of 37.000 cyclists on Norrebrogade on the bridge where the counter is placed was conducted in early September. We know from other surveys that the number of cyclists varies throughout the year. We also know that early September is a peak period for cyclists since the weather is often good and it is also the time of the year when the universities in the city start their academic year. Therefore many new students start to ride their bikes to and from the university in this period along with the many extra cyclists that only commute to work in the summer season. The number of 37.000 cyclists on the street is therefore representing the average number of daily cyclist on a normal work day in both directions on the street during the peak season. The actual number of cyclists counted manually in both directions during the 12 hours from 7AM to 7PM on the last official survey made on September 10th 2010 was 29.987. This number was corrected with the factor of 1.22 to cover the entire 24 hour period of traffic giving roughly the number of 37.000 cyclists in total daily on a normal working day in the peak season. On that particular day the automatic counter counted 16.265 cyclists over the entire 24 hours period in the outbound direction. In the interval between 7AM and 7PM it counted 12.106 cyclists, but the manual traffic count was 13.711 cyclists in the outbound direction only.
We are aware that the number of 37.000 cyclists has been reported in some media as the number of daily cyclists on the street on an average day throughout the year, but these media have failed to specify that this number represents the amount of cyclists on a normal working day in the peak season. The Department of Traffic has communicated about the project on Norrebrogade in many different contexts and sometimes we may not have stated clearly enough how we have arrived at the number of 37.000 cyclists.
However there is no doubt that there has been an increase of around 7000 cyclist daily on the street of Norrebrogade as a consequence of the project there, since the surveys of bicycle traffic before and after the project were made using the same method and the traffic was counted at same time of the year in the surveys before and after the project was initiated.
When it comes to the reliability of the automatic bicycle counter on the street it is also important to underline that we cannot fully trust the numbers counted as it is evident from the numbers shown above. The counter relies on a sensor placed below the asphalt on the bicycle path only. We know from our manual traffic surveys on the street that especially during the rush hour some faster cyclists overtake slower cyclists using the road, and some even the walkway, and thereby they avoid to be counted. We can also not fully trust that the counter registers all the cyclists when the bicycle traffic is dense as it is during the rush hour. It is our experience that the automatic counter generally counts around 10% less cyclists than the actual number during the peak hours and around 5% less in normal hours.
The idea of the counter is therefore not to be an official measurement, but more to give the cyclists an idea of the approximate magnitude of bicycle traffic on the street as they pass by and a feeling that each individual cyclist contributes to make Copenhagen a City of Cyclists.
I hope this explanation has clarified the matter. If you have any further questions please write again.
Sincerely
Klaus Grimar
Project Manager
Med venlig hilsen
Klaus Grimar
__________________________
KØBENHAVNS KOMMUNE
Teknik- og Miljøforvaltningen
Center for Trafik
Islands Brygge 37, 2.sal
Postboks 450
1505 Kbh. V