@La3per_l00k
If it's happening in all weathers search 'visualsettings.dat' for:

# emissive bits

& edit/lower the day values just under it

@La3per_l00k
If only happening in Overcast weather, search 'w_overcast.xml' for:

<light_vehicle_intenity_scale>

& lower the values for the times of day that are causing you trouble

@Jalice11
If you open the 'RAGEuphoria.oiv' or 'Weapon Physics (REQUIRED).oiv' with 7zip & have a look in the 'assembly.xml' files they will show you exactly what files were installed & where they were installed

If you installed them to the game folder, you'll have to follow @WGotch07's second suggestion & verify your files.

If you installed them to your 'mods' folder, you can use the original game folder vanilla files to replace the RAGEuphoria ones.

The RAGEuphoria '.oiv's install these files:

RAGEuphoria.oiv:

...mods\update\update.rpf\x64\data\tune\physicstasks.ymt
...mods\common.rpf\data\naturalmotion\alien.xml
...mods\common.rpf\data\naturalmotion\behaviours.xml
...mods\common.rpf\data\naturalmotion\fred.xml
...mods\common.rpf\data\naturalmotion\fred_large.xml
...mods\common.rpf\data\naturalmotion\taskmessages.xml
...mods\common.rpf\data\naturalmotion\wilma.xml
...mods\common.rpf\data\naturalmotion\wilma_large.xml
...mods\x64a.rpf\data\tune\physicstasks.ymt

Weapon Physics (REQUIRED).oiv:

...mods\update\update.rpf\common\data\ai\weapons.meta
...mods\common.rpf\data\ai\weapons.meta

...mods\update\update.rpf\dlc_patch\mpapartment\common\data\ai\weaponrevolver.meta
...mods\update\update.rpf\dlc_patch\mpbeach\common\data\ai\weaponsnspistol.meta
...mods\update\update.rpf\dlc_patch\mpbusiness\common\data\ai\weaponheavypistol.meta
...mods\update\update.rpf\dlc_patch\mpbusiness\common\data\ai\weaponspecialcarbine.meta
...mods\update\update.rpf\dlc_patch\mpbiker\common\data\ai\weaponminismg.meta
...mods\update\update.rpf\dlc_patch\mpbusiness2\common\data\ai\weaponbullpuprifle.meta
...mods\update\update.rpf\dlc_patch\mpgunrunning\common\data\ai\weapons_heavysniper_mk2.meta
...mods\update\update.rpf\dlc_patch\mphipster\common\data\ai\weaponvintagepistol.meta
...mods\update\update.rpf\dlc_patch\mpindependence\common\data\ai\weaponmusket.meta
...mods\update\update.rpf\dlc_patch\mplowrider\common\data\ai\weaponmachinepistol.meta
...mods\update\update.rpf\dlc_patch\mplowrider\common\data\ai\weaponmachinepistol.meta
...mods\update\update.rpf\dlc_patch\mplowrider2\common\data\ai\weaponcompactrifle.meta
...mods\update\update.rpf\dlc_patch\mpluxe\common\data\ai\weaponcombatpdw.meta
...mods\update\update.rpf\dlc_patch\mpvalentines\common\data\ai\weapongusenberg.meta

...mods\update\x64\dlcpacks\mpapartment\dlc.rpf\common\data\ai\weaponrevolver.meta
...mods\update\x64\dlcpacks\mpbiker\dlc.rpf\common\data\ai\weaponminismg.meta
...mods\update\x64\dlcpacks\mplowrider\dlc.rpf\common\data\ai\weaponmachinepistol.meta
...mods\update\x64\dlcpacks\mplowrider2\dlc.rpf\common\data\ai\weaponcompactrifle.meta
...mods\update\x64\dlcpacks\mpluxe\dlc.rpf\common\data\ai\weaponcombatpdw.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_bullpuprifle_mk2.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_doubleaction.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_marksmanrifle_mk2.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_pumpshotgun_mk2.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_revolver_mk2.meta
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_snspistol_mk2.meta    
...mods\update\x64\dlcpacks\mpchristmas2017\dlc.rpf\common\data\ai\weapons_specialcarbine_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_assaultrifle_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_carbinerifle_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_combatmg_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_heavysniper_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_pistol_mk2.meta
...mods\update\x64\dlcpacks\mpgunrunning\dlc.rpf\common\data\ai\weapons_smg_mk2.meta

You can do it manually if you like, but the quickest & easiest way to do it is to create your own '.oiv' uninstaller by using 7zip to replace the files in the original RAGEuphoria '.oiv's with their vanilla counterparts from your game folder (use Ctrl+F3 in OpenIV to find them quickly) & then reinstall that new edited '.oiv' to return the files to vanilla.
More info on how to do that here if you're interested.

@Mullac
No probs , I'm not too up on installing peds, but I'm pretty sure you need to have AddonPeds installed for that Superman to work.
AddonPeds has a couple of peds preinstalled ('vito_mafia' & a version of 'superman'). Concentrate on getting them working first, before trying to install any other additional peds

There is a 'Read Me.html' in the AddonPeds download, be sure to have a read of it & make sure you have all the prerequisites installed. Just be aware that AddonPeds was last updated 4 years ago (2017), so you may have to install versions of the prerequisites from 2017 (especially NativeUI (you probably want NativeUI v1.7 (released 29 Jun 2016) or perhaps v1.8 (released 8 Oct 2018)) to get it to work.

Be sure to have a read through all of the comments on the AddonPeds comments page (hit 'show previous 20 comments' button a heap of times to track through older comments) as people have posted links to certain solutions & youtube install vids that may help.

You are always going to run into some trouble installing older mods (especially if it's years old &/or the mod author has disappeared etc), there's no hard rules, what might work for others may or may not work for you. It'll just be all about getting as much information as you can from the community & trying each of the solutions to any issues you may face one by one until you possibly find something that works.

I can help you out & probably point you in the right direction, but it might be a good idea to start by installing a vehicle or two first, start with something simple until you get a feel for the game & how it's modded.

Note:
Not so important for add-on mods, but just for future reference etc, always, always, back up any '.rpf' archives before you edit & change them. Then you always have an out, & the ability to return to a working state of your modded game should something go wrong (sooner or later it will). Make sure it's a habit you get into

By all means, if you want, have a go at installing AddonPeds using the instructions in the 'Read Me.html' in the AddonPeds download & see what happens. See if you can get the included peds to work & then take it from there. If you hit any issues, give me a shout & we'll see what we can do about them one by one.

@Mullac
Are you installing the mods using OpenIV yeah? If so, to install all the files automatically the mod needs to be a '.oiv' installer (ie not '.rar' or '.zip').
If it's just files in a '.rar' or '.zip', you need to install them manually. If the mod has an '.oiv' installer (most don't), you extract the '.oiv' from within the '.rar' or '.zip' & then double-click the '.oiv' to install all files within it with OpenIV.
Link to one of the mods you are trying to install & having issues with & I'll take a look at it

@Scavenger23
This post explains the values, but you pretty much have all the important ones highlighted there

<swankness>:

The '<swankness>' value can affect frequency of spawns also, certainly on 'SWANKNESS_5'. That makes vehicles really rare to spawn, irrespective of other values & even works like that if a vehicle is the only one set to spawn on the streets, & even if it has a really high '<frequency value'.
Any spawn rate differences in 'SWANKNESS_1>4' are pretty hard to see, & possibly don't even have any effect (I never tested them to fully confirm, but it looked like they didn't have any effect on spawning).

<maxNum value=:

<maxNum value=1 should work & only allow one of that specific vehicle to spawn. That has always worked for me. Try setting it to '1' instead of '0'.

<frequency value=:

Have a look through your installed 'vehicles.meta' files & make sure the '<frequency' values are all roughly in the same range of numbers. I use values between 15 & 100, but there are no hard rules about it. Just make sure there are no odd crazy values in there.
For example, if most vehicles are set to 15>100, but one vehicle has a value of 20000 etc. That might cause some funky behaviour & unbalance the spawning system.

scripts:

Also, perhaps make sure you don't have any scripts that might be negatively affecting or messing with traffic spawning (rename 'scripts' folder (if exists) temporarily & test load game to confirm etc).

vehicles.meta location:

What 'vehicles.meta/s' are you editing? What are their locations in the game files?
You want to edit the main 'vehicles.meta' here:

...\mods\update\ update.rpf \common\data\levels\gta5\vehicles.meta

& for dlc vehicles, if more than one 'vehicles.meta' exists for any given dlc, you want to make sure you edit the ones in this folder structure:

...\mods\update\ update.rpf \ dlc_patch \

If for any given dlc, no 'vehicles.meta' exists within dlc_patch then you're safe to edit the ones in 'dlcpacks' folder structure here:

...\mods\update\x64\ dlcpacks \

settings.xml:

In 'settings.xml' here:

C:\Users\ YOUR WINDOWS USERNAME HERE \Documents\Rockstar Games\GTA V\settings.xml

there is a '<VehicleVarietyMultiplier value':

    <VehicleVarietyMultiplier value="1.000000" />

Make sure it is set to at least 1.000000, or perhaps test increasing the value further to see what difference it makes etc.

popgroups.ymt:

From a diagnostic point of view, it might be helpful to use 'popgroups.ymt' here:

...\mods\update\update.rpf\x64\levels\gta5\popgroups.ymt

to remove any vehicles you have added to traffic one by one (or in groups, whatever works for you) & confirm that they are all functioning correctly & none of them are messing with vehicle spawning. If one of your added vehicles is a bit funky, it might be messing the whole spawning system up & causing issues for other vehicles etc

gameconfig.xml:

The gameconfig.xml file has traffic related values in it. Not spawn frequency values per se, but traffic density & spacing values etc (all in the '<ConfigPopulation>' section).
Perhaps test out the vanilla version of the file just to rule it out etc.
If you have any issues with a vanilla gameconfig crashing the game, just use the vanilla '<ConfigPopulation>' section to replace the one in your modded gameconfig.
As always, make a backup of any files before you edit them.

Hopefully something there^ helps you out. Any specific questions or you run into any other issues, give me a shout

@GONZALO15000
Possibly a LOD issue. In 'vehicles.meta', pick a vehicle you are having issues with & then test setting these values & see if it fixes it?

vehicles.meta:

<lodDistances content="float_array">
        500.000000
        500.000000
        500.000000
        500.000000
        500.000000
        500.000000
      </lodDistances>

You can edit & optimise each '500' LOD level value after you confirm it works.
Basically, you want them as low as possible, but without any parts of vehicles disappearing.

@Doxxardu said in Vehicles.meta for all vehicles:

However, adding the same line to the vehicles.meta in this location:
mods\update\update.rpf\ dlc_patch \mpchristmas2018\common\data\levels\gta5

Yeah, that should definitely work

For other DLC's, you can even move '.meta' etc files over from the 'dlcpacks' &/or 'x64w.rpf' folder structures into 'dlc_patch' (in the correct place/folder structure of course) & they will load from there without having to edit 'update.rpf' in any other way, even if the files don't exist there originally

Example:
You can move the 'handling.meta', 'carcols.meta' & 'carvariations.meta' from here:

..\mods\update\x64\ dlcpacks \mpvinewood\dlc.rpf\common\data

to here (where they don't exist originally):

...\mods\update\update.rpf\ dlc_patch \mpvinewood\common\data

& they will now be the ones loaded by the game.

Handy to keep everything you edit in 'update.rpf' for ease of upgrading/updating/backing up etc

@Rizzx

outlines the basics, that's about all I know about scenarios.

@ReNNie @Doxxardu
Tested out a few different sort order values (highest/lowest etc), but didn't seem to make any difference.

I tried using 'vehicles.meta' data from another vehicle (that loads from 'update.rpf' 'vehicles.meta' no problem) in place of the italigto's. Same issue, so likely not a particular 'vehicles.meta' value causing it.

'handling.meta' data loads fine from 'update.rpf' 'handling.meta', so unrelated to that.
I also put the italigto's carvariations & carcols data into 'carvariations.ymt'/'carcols.ymt', but that didn't work either?!
Only variable appears to be loaded 'vehicles.meta' data location. When placed in 'update.rpf' 'vehicles.meta' game can't find vehicle or some part of it's setup it needs to spawn etc.

'mpchristmas2018' dlc set to 'EXTRACONTENT_COMPAT_PACK', so not sure why it can't find whatever it's missing?