The quick answer is ten to fifteen feet should be between each tunnel. However, below are some variables to consider when planning ahead on how to space your tunnels, especially if you want to set them up closer than the recommended spacing. 

Shadows cast from one tunnel to the next can reduce sunlight available for plant growth. This can be mitigated if the tunnels have a north-south orientation. However, if your growing context requires an east-west orientation (which is generally preferred), the lay of the land and the angle of the sun will need to be taken into consideration. Spacing between tunnels in relation to the sun is a concern due to shadows cast by the neighboring tunnel. This is more or less important in light of the growing season and crop selection. Ideally, tunnels are in an east to west orientation for solar gain. However, multiple variables must be considered. 

Snow also needs to be a thought for winter growers deciding how close their tunnels can be. When snow slides off the side of the tunnel it will build up on the tunnel walls. If the tunnels are too close, they will form a ditch-like-pocket where snow will collect. The distance between is relative to the amount of snow in a typical year. The greater amounts of snow, the greater the distance needs to be. Any snow that is removed has nowhere else to go. Depending on snowfall amounts and machinery size, one should count on at least 8-15 feet between tunnels so they can remove the snow from the side. You may find this article on snow useful as well. 

Rain is an additional variable to take into account, as the tunnels create a watershed. Thus causing issues with flooding or seed erosion for direct-seeded crops on outer rows. Soil type, lay of the land, and the typical rain intensity will play a role in this decision. With soils that absorb water better and areas that have less intense rain, the tunnels can be placed closer together.  Additionally, growing a cover crop in-between will help with water absorption and weed management in situations where they are closer than 30-inches or less.

Orientation (N-S, E-W) in relation to the lay of the land is something else to think about. Typically, we recommend tunnels follow the lay of the land lengthwise. In other words, like the caterpillar is inching up the slant, not across the slant. 

Wind passing over and around structures can create a wind tunnel effect. This can increase winds well in excess of the actual wind speed. Spacing out tunnels or creating windscreens can help with this. 

Spacing between tunnels depends on local weather context and tunnel orientation. Generally speaking, tunnels should be at least a tunnels height away or 8-10 feet, but 10-15 feet is ideal. However, they can be closer.