We provide necessary and sufficient conditions on the density \(W:\mathbb {R}^d\times \mathbb {R}^d\rightarrow \mathbb {R}\) in order to ensure the sequential weak \(^*\) lower semicontinuity of the functional \(J: W^{1,\infty }(I;\mathbb {R}^d)\rightarrow \mathbb {R}\) , defined as \(\begin{aligned} J(u):=\underset{I\times I}{\mathrm {ess\,sup}}\,W(u'(x), u'(y)), \end{aligned}\) when I is an open and bounded interval of \(\mathbb {R}\) . We also show that, when \(d=1\) , the lower semicontinuous envelope of I in general can be obtained by replacing W by its separately level convex envelope.