I could not help myself: this is just a spin on egreg's answer with some finetuning, while not changing anything profound:
- Streamlined legend (
R: RF instead of (1) RF)
- Grouped columns with accordingly shortened midrules

\documentclass[runningheads]{llncs}
\usepackage{booktabs,siunitx}
\begin{document}
\title{Paper Title}
\begin{table}[htp]
\caption{Classification results of imbalance techniques}
\setlength{\tabcolsep}{0pt}
\begin{tabular*}{\textwidth}{
@{\extracolsep{\fill}}
l
@{\hskip 1ex}
*{5}{S[table-format=1.2]}
@{\hskip 1ex}
*{5}{S[table-format=1.2]}
@{\hskip 1ex}
*{5}{S[table-format=1.2]}
@{}
}
\toprule
TM &
\multicolumn{5}{c}{Precision} &
\multicolumn{5}{c}{Recall} &
\multicolumn{5}{c}{F1-Score} \\
\cmidrule(r{1ex}){2-6} \cmidrule(r{1ex}){7-11} \cmidrule{12-16}
& {R} & {S} & {A} & {B} & {D} & {R} & {S} & {A} & {B} & {D} & {R} & {S} & {A} & {B} & {D}\\
\midrule
Walk & 0.88 & 0.78 & 0.65 & 0.59 & 0.45 & 0.38 & 0.73 & 0.95 & 0.83 & 0.57 & 0.52 & 0.07 & 0.22 & 0.66 & 0.88 \\
Bike & 0.88 & 0.78 & 0.65 & 0.59 & 0.45 & 0.38 & 0.73 & 0.95 & 0.83 & 0.57 & 0.52 & 0.07 & 0.22 & 0.66 & 0.88 \\
\bottomrule
\end{tabular*}
\medskip
TM: Transportation mode
R: RF; S: RF-SMOTEd; A: AdaBoost; B: SMOTEBoost; D: DECOC
\end{table}
\end{document}
And here my final take - the cramped look did not let me rest:
- Switched to
% as the unit, dropping all 0.
- Transposed the axes: Transportation mode and method swapped
- Top left corner label switched from "mode" to "method". If not clear from context, the meaning from "walk" and "bike" could be made clear in the caption.

\documentclass[runningheads]{llncs}
\usepackage{booktabs,siunitx}
\begin{document}
\title{Paper Title}
\begin{table}[htp]
\centering
\caption{Classification results of imbalance techniques (\%)}
\begin{tabular}{
@{\extracolsep{\fill}}
l
*{2}{S[table-format=2]}
*{2}{S[table-format=2]}
*{2}{S[table-format=2]}
@{}
}
\toprule
&
\multicolumn{2}{c}{Precision} &
\multicolumn{2}{c}{Recall} &
\multicolumn{2}{c}{F1-Score} \\
\cmidrule(r{1ex}){2-3} \cmidrule(r{1ex}){4-5} \cmidrule{6-7}
Method & {Walk} & {Bike} & {Walk} & {Bike} & {Walk} & {Bike} \\
\midrule
RF & 88 & 88 & 38 & 88 & 52 & 88 \\
RF-SMOTEd & 78 & 78 & 73 & 78 & 07 & 78 \\
AdaBoost & 65 & 65 & 95 & 65 & 22 & 65 \\
SMOTEBoost & 59 & 59 & 83 & 59 & 66 & 59 \\
DECOC & 45 & 45 & 57 & 45 & 88 & 45 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}