 Complete the following phrase:  For London Underground passengers, research suggests that transfers are highly costly in terms of walk and wait times. Because these costs are unevenly distributed across stations and platforms, path choice analyses may be helpful in guiding upgrades and choice of new stations. Routes on the Underground can also be optimized using a global network optimization approach, akin to routing algorithms for Internet applications. Analysis of the Underground as a network may also be helpful for setting safety priorities, since the stations targeted in the 