Travel control device, travel control method, and storage medium storing travel control program

A travel control device comprising:

• a computer;
• and
• a non-transitory storage device,
• wherein
• the computer, by executing a control program stored in the non-transitory storage device, is configured to execute:
• a coasting travel process;
• a cruise travel process;
• a gradient information acquisition process;
• a deviation variable acquisition process;
• and
• a switching process,
• the coasting travel process performs a coasting travel of a vehicle,
• the cruise travel process controls a speed of the vehicle to a set vehicle speed while giving a priority to satisfaction of a predetermined condition,
• the predetermined condition is a condition for maintaining a distance to a preceding vehicle within a predetermined distance range when the preceding vehicle is located within a predetermined range ahead of the vehicle,
• the gradient information acquisition process acquires, as gradient information, information indicating a gradient of a road surface on which the vehicle is traveling,
• the deviation variable acquisition process acquires, separately from the gradient information, a deviation variable for determining an acceleration generated in the vehicle when the vehicle travels, by performing the coasting travel process, a downward slope having a predetermined gradient,
• the switching process uses the gradient information and the deviation variable as inputs and performs switching to the coasting travel process when the vehicle travels the downward slope while the cruise travel process is being executed, the switching process performs switching to the coasting travel process according to a value of the deviation variable even though a magnitude of the gradient of the road surface is smaller than a reference value,
• the deviation variable includes a variable indicating whether the vehicle is traveling in a one-way tunnel, and
• the switching process performs switching to the coasting travel process when the deviation variable indicates that the vehicle is traveling in the one-way tunnel, even though the magnitude of the gradient of the road surface is smaller than the reference value set for a traveling outside the one-way tunnel.