Travel assistance method, road captured image collection method, and roadside device

Provided is a system which can reduce load on a travel controller that performs a danger avoidance operation by preliminarily detecting a significant event without increasing an amount of ITS communications, thereby providing suitable assistance for travel control of autonomous vehicles. When a vehicle 1 passes through a certain section of a road after an in-vehicle terminal 4 has detected a significant event while the vehicle 1 is traveling in the section, the in-vehicle terminal 4 transmits event data (images and other information related to the event) to a roadside apparatus 6 at an end point of the section. The roadside apparatus 6 transmits the event data to another roadside apparatus 6 at a start point of the section, which in turn transmits the event data to an in-vehicle device in another vehicle entering the section so that the device can perform travel control based on the event data.