解：?$(1)$?設(shè)?$CD$?段的函數(shù)表達式為?$y=kx+b$?
∵?$AD=5.5m，$??$AD：$??$AC=1：$??$4$?
∴?$AC=22m$?
將點?$(15，$??$5.5)、$??$(37，$??$0)$?代入表達式得?$\begin{cases}{k=-0.25}\\{b=9.25}\end{cases}$?
∴函數(shù)表達式為?$y=-0.25x+9.25(15≤x≤37)$?
令?$y=4，$?則?$x=21$?
?$AB=A'B'=21-15=6m$?
?$(2)CD$?段的函數(shù)表達式為?$y=-0.25x+9.25(15≤x≤37)$?
?$CD$?和?$C'D'$?關(guān)于?$y$?軸對稱，
則?$C'D'$?段的函數(shù)表達式為?$y=0.25x+9.25(-37≤x≤-15)$?
設(shè)?$D'FD$?段的函數(shù)表達式為?$y=ax^2+8$?
將點?$(15，$??$5.5)$?代入得?$a=-\frac {1}{90}$?
?$D'FD$?段的函數(shù)表達式為?$y=-\frac {1}{90}x^2+8(-15＜x＜15)$?
?$(3)$?在?$D'FD$?段，令?$x=2，$??$y=\frac {358}{45}≈7.96$?
?$7.96m＞7.4m$?
∴能從橋下安全通行
