My MWE is adapted from How to diagonally divide a table cell … properly?.
Two questions:
- I add an empiric rule of
0.71\baselineskip, but why0.71? - Why
c=...is not centered as the text below is?
Edit One ca see a simplified and corrected version here : Optional argument problem

\documentclass[10pt,french]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{array}
\usepackage{calc}
\usepackage{tikz,fourier}
\newcommand\diag[5]{%
\multicolumn{1}{|m{#2}|}{\hskip-\tabcolsep
$\vcenter{\begin{tikzpicture}[baseline=0,anchor=south west,outer sep=0]
\path[use as bounding box] (0,0) rectangle (#2+2\tabcolsep,\baselineskip);
\node[minimum width={#2+2\tabcolsep-\pgflinewidth},
minimum height=\baselineskip+#3-\pgflinewidth] (box) {};
\draw[line cap=round] (box.north west) -- (box.south east);
\node[anchor=south west,align=left,inner sep=#1] at (box.south west) {#4};
\node[anchor=north east,align=right,inner sep=#1] at (box.north east) {#5};
\end{tikzpicture}}\rule{0pt}{.71\baselineskip+#3-\pgflinewidth}$\hskip-\tabcolsep}}
\begin{document}
\begin{tabular}{%
|>{\centering\arraybackslash}m{4.5cm}
|*{4}{>{\centering\arraybackslash}m{2.5em}|}}\hline
\diag{\tabcolsep}{4.5cm}{3.4em}{Coefficient\\ de frottement $c$}{Vitesse $V$\\ (en km/h)}&50&90&110&20\\ \hline
$c=0,8$\newline route s\`eche, bitume neuf &&&&\\ \hline
$c=0,7$\newline route s\`eche, bitume moyen &&&&\\ \hline
$c=0,8$\newline route mouill\'ee, bitume moyen &&&&\\ \hline
\end{tabular}
\end{document}


0.71be an approximation to1/\sqrt{2}=0.70710678...? – Mico May 05 '14 at 11:43\strutdefinition is\rule[-.3\baselineskip]{0pt}{\baselineskip}, I thaught it came from hidden\strut... – Tarass May 05 '14 at 16:02