Function

When placing two connecting underground belts, it will consume transport belts equal to the gap between them. Transport belts can be taken from the player's inventory or delivered by bots. They will be refunded when deconstructed or upgraded.
Underground belt recipes are much cheaper, and stack higher.
The effect of this system is that underground belts are more expensive when built longer, however on average it will cost the same or less.
As an added bonus, there is no need for an arbitrary length limit, so max length can be set as high as you want without disrupting the game balance.

Purpose

I made this mod because I always feel like I am playing sub-optimally when I do not build undergrounds to their max length. It especially hurts me when I use the upgrade planner to upgrade yellow undergrounds, or place unconnected undergrounds as part of a belt balancer, knowing how much iron is being wasted.

Recipe Tweaks (optional)

2x underground - 3 transport belts, 6 iron plates
2x fast underground - 2 undergrounds, 15 iron gears
2x express underground - 2 fast undergrounds, 30 iron gears, 60 lubricant
2x turbo underground - 2 express undergrounds, 15 tungsten plates, 60 lubricant
With these recipes, placing max length underground belts will be slightly more expensive, but building them shorter will be much cheaper.
For modded belts, it will attempt to automatically adjust the recipe. May have mixed results.

Stack Size (optional)

Underground belt stack size is doubled by default.

Configurable Length

For players who love long underground belts, the max length of every underground can be increased up to 30.
This would cover a full factorio chunk (32 tiles, if you include the underground belts + 30 tile gap)
There is no need to worry about carrying multiple items for each length of underground. Just build as long as needed.

Compatibility / Issues

This mod is experimental and may break in certain situations
Compatible with Space Age DLC
Compatible with most modded belts
If you run out of transport belts, it will not stop you from placing undergrounds. Any missing belts will be requested by bots.
Untested with quality. Probably doesn't work. Best to avoid using quality underground belts.
Try not to use the upgrade planner on a belt-weave. It is unstable.