  We present Arc-Flag TB, a journey planning algorithm for public transit
networks which combines Trip-Based Public Transit Routing (TB) with the
Arc-Flags speedup technique. Compared to previous attempts to apply Arc-Flags
to public transit networks, which saw limited success, our approach uses
stronger pruning rules to reduce the search space. Our experiments show that
Arc-Flag TB achieves a speedup of up to two orders of magnitude over TB,
offering query times of less than a millisecond even on large countrywide
networks. Compared to the state-of-the-art speedup technique Trip-Based Public
Transit Routing Using Condensed Search Trees (TB-CST), our algorithm achieves
similar query times but requires significantly less additional memory. Other
state-of-the-art algorithms which achieve even faster query times, e.g., Public
Transit Labeling, require enormous memory usage. In contrast, Arc-Flag TB
offers a tradeoff between query performance and memory usage due to the fact
that the number of regions in the network partition required by our algorithm
is a configurable parameter. We also identify an issue in the transfer
precomputation of TB that affects both TB-CST and Arc-Flag TB, leading to
incorrect answers for some queries. This has not been previously recognized by
the author of TB-CST. We provide discussion on how to resolve this issue in the
future. Currently, Arc-Flag TB answers 1-6% of queries incorrectly, compared to
over 20% for TB-CST on some networks.
