Best to travel here out of season. The road up the volcano is apparently gridlocked in high season. We went in November and there was no traffic at all. The final walk from the carpark to the crater rim is quite strenuous and takes 20-30 minutes. Buses from the Circumvesuviana Pompeii Rail Station do not run out of season. There is however a taxi bus service. Make up a party of minimum 4 people and pay no more than Euro 15 per person (there are always other people waiting to make up a party) This service is only Euro 2 pp more expensive than the bus when you take the site entry fee into account.
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