$解：如圖，根據(jù)題意，知CD//AB//MN$
$∴△CDE∽△ABE，△MNF∽△ABF$
$∴\frac {CD}{AB}=\frac {DE}{BE}，\frac {MN}{AB}=\frac {FN}{FB}$
$∵CD=1.8m，DE=1.8m，MN=1.5m，F(xiàn)N=1.5m，DN=2.7m$
$∴\frac {1.8}{AB}=\frac {1.8}{1.8+BD}，\frac {1.5}{AB}=\frac {1.5}{1.5+2.7-BD}$
$∴易得AB=3m$
$∴路燈的高為3m$
