The road M-501a, was a road of the Community of Madrid and was the result of the change of layout of the M-501 road and the failed passage of this stretch to the City Council of Alcorcón during the government in the town of Pablo Zúñiga . The City Council began the unfolding from kilometer 1 to the M-50 but without finishing these works. When arriving at the power Enrique Cascallana denounces that the City council is not nor has never been owner of this way and renounces to continue with the works. While the legal issue is resolved the Community of Madrid had to do some emergency works.
Finally, the road passes through a judicial decision to be property of the Community of Madrid, which passes it without much meaning M-501a.
In 2016 and due to the opening of the Parque Lucero shopping center (at the end of this road), its entire route is limited to 50 km / h. In addition, this road no longer appears in the catalog of roads of the Community of Madrid so its official name is that of Avenida de San Martín de Valdeiglesias.
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