what mod can be done so that the added cars are in traffic

daylaynann

the same with the police, so that the cops ride new cars

a63nt-5m1th

For cars in traffic:

You can add them to one of the vehicle groups within the '<vehGroups>' section in 'popgroups.ymt' & they will spawn in traffic:

...\Grand Theft Auto V\mods\update\update.rpf\x64\levels\gta5\popgroups.ymt

Example: (popgroups.ymt)

  <vehGroups>
    <Item>
      <Name>VEH_POOR</Name>
      <models>
        <Item><Name>asea</Name><Variations type="NULL"/></Item>
        <Item><Name>buccaneer</Name><Variations type="NULL"/></Item>
        <Item><Name>intruder</Name><Variations type="NULL"/></Item>
        <Item><Name>ingot</Name><Variations type="NULL"/></Item>
        <Item><Name>manana</Name><Variations type="NULL"/></Item>
        <Item><Name>premier</Name><Variations type="NULL"/></Item>
        <Item><Name>regina</Name><Variations type="NULL"/></Item>
        <Item><Name>asterope</Name><Variations type="NULL"/></Item>
        <Item><Name>stratum</Name><Variations type="NULL"/></Item>
      </models>
      <flags>POPGROUP_SCENARIO POPGROUP_AMBIENT</flags>
    </Item>
    <Item>
      <Name>VEH_MID</Name>
      <models>
        <Item><Name>buffalo</Name><Variations type="NULL"/></Item>
        <Item><Name>gresley</Name><Variations type="NULL"/></Item>
        <Item><Name>Dubsta</Name><Variations type="NULL"/></Item>
        <Item><Name>dilettante</Name><Variations type="NULL"/></Item>
        <Item><Name>fusilade</Name><Variations type="NULL"/></Item>
        <Item><Name>gauntlet</Name><Variations type="NULL"/></Item>
        <Item><Name>landstalker</Name><Variations type="NULL"/></Item>
        <Item><Name>primo</Name><Variations type="NULL"/></Item>
        <Item><Name>oracle</Name><Variations type="NULL"/></Item>
        <Item><Name>peyote</Name><Variations type="NULL"/></Item>
        <Item><Name>sultan</Name><Variations type="NULL"/></Item>
        <Item><Name>stanier</Name><Variations type="NULL"/></Item>
        <Item><Name>washington</Name><Variations type="NULL"/></Item>
      </models>
      <flags>POPGROUP_NETWORK_COMMON POPGROUP_SCENARIO POPGROUP_AMBIENT</flags>
    </Item>
    <Item>
      <Name>VEH_RICH</Name>
      <models>
        <Item><Name>bullet</Name><Variations type="NULL"/></Item>
        <Item><Name>coquette</Name><Variations type="NULL"/></Item>
        <Item><Name>f620</Name><Variations type="NULL"/></Item>
        <Item><Name>ninef</Name><Variations type="NULL"/></Item>
		...
Note: More sections below this in 'popgroups.ymt'

For dispatch:

You can add vehicles to any '<VehicleModels>' section within the '<VehicleSets>' section of 'dispatch.meta' & they will spawn as dispatch:

...\Grand Theft Auto V\mods\update\update.rpf\common\data\dispatch.meta

Example: (dispatch.meta)

	<VehicleSets>
		<Vehicle>
			<Name>POLICE_CAR</Name>
			<ConditionalVehicleSets>
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
					<VehicleModels>
						<Vehicle>crusader</Vehicle>
						<Vehicle>mesa3</Vehicle>
						<Vehicle>barracks</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_M_Marine_01</Ped>
						<Ped>S_M_Y_Marine_01</Ped> 						
						<Ped>S_M_Y_Marine_03</Ped> 				
					</PedModels>
				</Item>
				<Item>
					<ZoneType>VEHICLE_RESPONSE_COUNTRYSIDE</ZoneType>
					<VehicleModels>
						<Vehicle>SHERIFF</Vehicle>
						<Vehicle>SHERIFF</Vehicle>
						<Vehicle>SHERIFF</Vehicle>
						<Vehicle>SHERIFF2</Vehicle>						
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Sheriff_01</Ped> 				
					</PedModels>
				</Item>
				<Item>
					<VehicleModels>
						<Vehicle>police3</Vehicle>
						<Vehicle>police2</Vehicle>
						<Vehicle>police</Vehicle>
						<Vehicle>policet</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 				
					</PedModels>
				</Item>		
			</ConditionalVehicleSets>
		</Vehicle>
		...
Note: Additional sections below this in 'dispatch.meta' also.

dispatch.meta '<ZoneType>' Info:

There are 3 main ZoneTypes within the '<VehicleSets>' section in 'dispatch.meta', they are:

VEHICLE_RESPONSE_ARMY_BASE:
These are the dispatch vehicles that spawn at Zancudo Air Base
VEHICLE_RESPONSE_COUNTRYSIDE:
These are the dispatch vehicles that spawn out in the Countryside
No '<ZoneType>' line/name:
These are the dispatch vehicles that spawn in the main City area of the game

'FLAG_LAW_ENFORCEMENT' flag required in 'vehicles.meta' for dispatch to spawn correctly:

Also, for dispatch, remember to add a 'FLAG_LAW_ENFORCEMENT' flag to the '<flags>' line within 'vehicles.meta' for each vehicle you add, so that they can spawn correctly as dispatch (no unresponsive driverless ghost dispatch vehicles etc).

Example: (vehicles.meta)

         <flags>FLAG_LAW_ENFORCEMENT FLAG_EMERGENCY_SERVICE FLAG_NO_RESPRAY FLAG_DONT_SPAWN_IN_CARGEN FLAG_REPORT_CRIME_IF_STANDING_ON FLAG_HAS_LIVERY FLAG_HAS_INTERIOR_EXTRAS</flags>

Sata

@daylaynann if you're lazy there's Added Traffic

It does work, and the author included a tool that is simple to use and gave me a good initial understanding on how zones are defined across the map.

It can cause performance issues tho, specially with fancy add-on cars with limitless polygons. It will still decrease performance anyway even with vanilla as it'll be deleting/creating vehicles all the time. I've opted to add DLC cars in traffic for variety, just like @a63nt-5m1th suggested.