TxDOT MS4 boundaries - Municipal Separate Storm Sewer System (MS4) permit requirements were implemented in two phases: Phase I for large and medium MS4s and Phase II for regulated small MS4s.
<p><font size='2' style='font-weight:normal;'></font></p><p><span style='font-size:small; font-weight:normal;'>Phase I MS4 Requirements</span><br /></p>

<p><font size='2' style='font-weight:normal;'>If you operate an MS4 which serves or is located in an
incorporated place or county with a population of 100,000 or greater (based on
the 1990 U.S. Census), then you operate a regulated Phase I MS4 and you must
apply for an individual permit*.</font></p>

<p><font size='2' style='font-weight:normal;'> </font><span style='font-weight:normal; font-size:small;'>*Please note that all Phase I MS4s are already permitted and
there are no new Phase I designations.</span></p>

<p><font size='2' style='font-weight:normal;'>Phase II MS4 Requirements</font></p>

<p><font size='2' style='font-weight:normal;'>If you operate a small MS4:</font></p>

<p><font size='2' style='font-weight:normal;'>Located in an urbanized area (UA), as defined
by the Bureau of the Census (note that MS4s may be regulated based on the 2000
or the 2010 Census UAs), or </font><span style='font-weight:normal; font-size:small;'>located outside of a UA and are brought into the program on a
case-by-case basis (i.e., “designated”) by the TCEQ, </span><font size='2'> </font><span style='font-weight:normal; font-size:small;'>then you operate a regulated small MS4 and must obtain
authorization or apply for a waiver.</span></p>
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