Studies on bicycle path designs and risks analysis

Posted by concerned citizens : 

Since 2000 Polk Street has had sharrows from Post Street north to Union Street and designated bike lanes from Post Street south to McAllister.  According to the SFMTA, over the last 5 years or so there have been twice the number of bike accidents on lower Polk where there are designated bike lanes than on upper Polk where there are sharrows:

Geary to McAllister (6 blocks)  37 bike accidents
Clay to Geary (7 blocks) 18 bike accidents
Union to Clay (7 blocks) 14 bike accidents

Most bicycle accidents in San Francisco, including Polk Street, occur at intersections.  Will the addition of a designated bike lane from California Street north to Union Street and a raised cycle track from California Street south to McAllister Street decrease or increase the number of accidents on Polk Street?  Where is the research to show the SFMTA’s new plans will improve bike safety on Polk Street?

Wikipedia:  Segregated Cycle Facilities

Cycle path collision risks:

Studies showing greater benefits:

A large study undertaken by S.U. Jensen et al.[35][36] into the safety of Copenhagen cycle tracks before and after they were constructed concludes “The construction of cycle tracks in Copenhagen has resulted in an increase in cycle traffic of 18–20% and a decline in car traffic of 9–10%. The cycle tracks constructed have resulted in increases in accidents and injuries of 9–10% on the reconstructed roads. The increase of accidents and injuries increased at intersections while decreased mid-block.”

Studies showing greater risks:

A Danish study by Agerholm et al. in 2008[50] concluded that “Through the years many studies have shown that bicycle paths in built-up areas impair traffic safety. A new Danish study presented in this article confirms these results… the main results are that bicycle paths impair traffic safety and this is mainly due to more accidents at intersections, and that there has been no improvement in the design of new bicycle paths compared to the older ones.” . . .

A statistically significant increase in the total number of injury accidents by 10% was found. It is mainly caused by a significant increase of 18% in the number of injury accidents in intersections.

Why would the SFMTA want to implement a design that has been shown to increase intersection bicycle accidents?  If bicycle accidents increase, will the SFMTA have an excuse to build more bicycle infrastructure?  See,  

Should the MTA install cycle tracks that give unskilled bicyclists a greater perception of safety but which are known to increase the risk of intersection accidents?

Should the City and County of San Francisco be held liable for intersection accidents arising from this design defect?