This style can be used to highlight the unnammed highways.
{{{
#!style type="mapcss"

meta 
{
    title: "Streets Have No Name";
    description: "Could be a U2 song: highlights where the streets have no name";
    version: "0.[[revision]]_[[date]]";
    author: "Yann Coupin";
}

meta[lang=fr]
{
    title: "Sans nom";
    description: "Ça pourrait être une chanson de U2: « Where the streets have no name », surligne les rues sans nom";
}

way[highway][!name]
{
    width: 3;
    dashes: 10,10;
    dashes-background-color: red;
}

}}}