We present the first approximate distance oracle for sparse directed networks with time-dependent arc-travel-times determined by continuous, piecewise linear, positive functions possessing the FIFO property. Our approach precomputes (1 + ε)-approximate distance summaries from selected landmark vertices to all other vertices in the network, and provides two sublinear-time query algorithms that deliver constant and (1 + σ)-approximate shortest-travel-times, respectively, for arbitrary origin-destination pairs in the network. Our oracle is based only on the sparsity of the network, along with two quite natural assumptions about travel-time functions which allow the smooth transition towards asymmetric and time-dependent distance metrics.
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