The CTA this month is experiencing the most slow zones the system has seen in more than four years, according to recently released data.

Nearly 16 percent of the system is under slow zone, during which trains operate at 35 miles per hour or slower, according to a map posted on transitchicago.com last week. This is the highest percentage of slow zones since April 2008, when 16.1 percent of the system was under slow zone, CTA data shows.

This month, there's at least one slow zone on every rail line. The zones are the worst in the Loop and on the Red, Purple, Brown and Green Lines, which are each seeing more than a fifth of their track under slow zone.

The CTA is working on eliminating the slow zones on the northern Red Line, where seven stations are undergoing rehabilitation this year.

The agency will tackle the slow zones on the southern portion of the Red Line next year when it shutters nine South Side stations for five months.

On weekends this year, the CTA has been working on upgrading its signal system in the Loop.

And although viaducts were replaced earlier this year in Evanston, some Purple Line slow zones still persist. The CTA does not have a date set for improvement of this track.