What is my brother?

Question

My little brother likes puzzles a lot, and recently he started making his own.

The first ones were simple, but his newest got me good. He always describes one word with simple but not too revealing hints. One per day.

1. I can be found outside of cities.
2. In winter I freeze and become slippery.
3. Building one of me is rather difficult because it is done in a forest where the ground is muddy.
4. You can find a lot of me spread across the country.
5. You drive over me and not through me.
6. The air is cleaner around me.

Do you know what my brother is? He gives me a new hint every day.

New Hint: 7. You need me when there is too much traffic.

Answer 1

Is he a

Forest Path

I can be found outside of cities.

they mostly exist in Forests outside of cities

In winter I freeze and become slippery.

they are not getting cleared by Snowplows so they are slippery in winter

Building one of me is rather difficult because it is done in a forest where the ground is muddy.

they are built in Forests and building them can be hard when it's too muddy

You can find a lot of me spread across the country.

They exist everywhere in all Forests

You drive over me and not through me.

it's a path, so you can only drive on it

The air is cleaner around me.

they exist in Forests, where the air is cleaner

You need me when there is too much traffic.

when there is too much traffic, you can use them as shortcut/bypass

Answer 2

Are you a:

golf course?

I can be found outside of cities.

Probably don't want to break any windows

In winter I freeze and become slippery.

You can have frost, and there can be ponds

Building one of me is rather difficult because it is done in a forest where the ground is muddy.

Golf courses are often built in or next to forests

You can find a lot of me spread across the country.

Maybe in some countries, or perhaps by country you mean countryside

You drive over me and not through me.

You drive golf balls over golf courses

The air is cleaner around me.

Less pollution

Answer 3

Are you

a CANAL?

I can be found outside of cities.

Usually.

In winter I freeze and become slippery.

If it's cold enough.

Building one of me is rather difficult because it is done in a forest where the ground is muddy.

Canals, unlike rivers, are built. They might also run through forests.

You can find a lot of me spread across the country.

Depends which country, but certainly in some countries this is true.

You drive over me and not through me.

You can drive across a bridge over a canal, but you can't drive through it (at least not in a car).

The air is cleaner around me.

Probably?

Answer 4

I'm going to go with a

RAILROAD

I can be found outside of cities.

Usually connects cities

In winter I freeze and become slippery.

(metal) RR tracks and (wooden) RR ties are pretty slippery in the cold

Building one of me is rather difficult because it is done in a forest where the ground is muddy.

Edit: Major landscaping needs to be done to create a sturdy and fairly level path through unstable surfaces like muddy forests

You can find a lot of me spread across the country.

RR are spread throughout many countries

You drive over me and not through me.

You drive over the RR tracks

The air is cleaner around me.

In the forests or outside of the polluted cities

New Hint: 7. You need me when there is too much traffic.

An alternative to automobiles

Answer 5

I think you might be a

Pond

It is pretty similar towards @Rand al'Thor but with hint 7 in mind:

I can be found outside of cities.

Must of the time they are located outside the city.

In winter I freeze and become slippery.

A pond is usually not that big and can easily freeze in the winter

Building one of me is rather difficult because it is done in a forest where the ground is muddy.

A lot of ponds are located in forests and the terrain is muddy around it.

You can find a lot of me spread across the country.

Most countries have a lot of them

You drive over me and not through me.

You use a bridge to get over it, although some people have tried to go through it by car before but it mostly didn't end well.

The air is cleaner around me.

Less pollution in remote areas

You need me when there is too much traffic.

depending on which country you are you use the currency pond to pay for a tollway and get out of traffic.