Where is the coach going, the one in which Mr. Lorry is riding? Why are the passengers walking? (Book 1 Chapters 1-6)

Mr. Jarvis Lorry, a confidential clerk at Tellson's Bank of London, is on his way to Dover in a mail-coach. It is a cold night and he is wrapped up to the ears, so his physical appearance is concealed from his fellow-passengers, all of whom are strangers. The coachman fears his passengers just as they fear one another.Some walk because coaches are frequently attacked.