Some people were asking me if I could modify pursuing/response vehicles too in traffic. Here's an answer.

This readme shows you how to add vehicles into dispatch.meta.

For example, you want to add the Aventador Police Car by Shimonbz3 as one of the police cars that will
chase you in wanted levels.
The vehicle had its internal name 'police5' as the add-on.

What you should do:

Find this line in the POLICE_CAR group:
				<Item>
					<VehicleModels>
						<Vehicle>police3</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
				</Item>

Add this under it:
				<Item>
					<VehicleModels>
						<Vehicle>police5</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
				</Item>
Save, replace the dispatch.meta, open the game, and get a wanted level. You will see one now chasing
you down.
Note that the 'police5' can be any vehicle, depends on the vehicle you're adding.

Here's the list of groups available in dispatch.meta and what were their uses:
This line (VEHICLE_RESPONSE_ARMY_BASE):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
					<VehicleModels>
						<Vehicle>crusader</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_M_Marine_01</Ped> 
					</PedModels>
				</Item>
This line was used for patrol cars on Fort Zancudo ranges and the vehicles listed were used to chase you down when you got a wanted level near Fort Zancudo.

This line (VEHICLE_RESPONSE_COUNTRYSIDE):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_COUNTRYSIDE</ZoneType>
					<VehicleModels>
						<Vehicle>SHERIFF</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Sheriff_01</Ped> 
					</PedModels>
				</Item>
This line was used for police cars that were regularly found patrolling Blaine County, and the vehicles listed were used to chase you down when you got a wanted level in Blaine County.

This line: (POLICE_CAR), under (VEHICLE_RESPONSE_COUNTRYSIDE):
				<Item>
					<VehicleModels>
						<Vehicle>police3</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
				</Item>
This line was used for police cars that were found in Los Santos (and especially near the Legion Square and Mission Row), and the vehicles listed were used to chase you down when you got a wanted level in Los Santos.

This line (POLICE_MOTORCYCLE):
				<Item>
					<VehicleModels>
						<Vehicle>policeb</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
				</Item>
This line was the list of motorcycles that were used by the SAHP that can be sometimes found around Blaine County randomly. Can be used to chase the player down in wanted levels along with regular police cars.

This line (SWAT_JEEP):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
					<VehicleModels>
						<Vehicle>crusader</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_M_Marine_01</Ped> 
					</PedModels>
				</Item>
				<Item>
					<ZoneType>VEHICLE_RESPONSE_COUNTRYSIDE</ZoneType>
					<VehicleModels>
						<Vehicle>sheriff2</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Swat_01</Ped> 
					</PedModels>
				</Item>
				<Item>
					<VehicleModels>
						<Vehicle>fbi2</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Swat_01</Ped> 
					</PedModels>
				</Item>
These were the vehicles used to chase down the player when having a wanted level of 4 or more. Note that the vehicles listed in the VEHICLE_RESPONSE_ARMY_BASE group were the vehicles used when near Fort Zancudo, vehicles listed in
the VEHICLE_RESPONSE_COUNTRYSIDE group were the vehicles used in Blaine County area, and the one with no zone type were the vehicles used in Los Santos area.

This line (POLICE_HELI_1):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
				</Item>
				<Item>
					<VehicleModels>
						<Vehicle>polmav</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Swat_01</Ped> 
					</PedModels>
				</Item>
These were the helicopters used to join the pursuit when  the player had a wanted level of 3 or more and can randomly be seen around San Andreas. There is also a POLICE_HELI_2 group line that were unused, and the line implies that the
Annihilator will be seen pursuing the player when near Fort Zancudo, with NOOSE members aboard. It's your own risk to modify it, I haven't tested it yet.
Note: The POLICE_HELI_2 line were unused which also makes the fact that the Annihilator becomes a GTA Online-only exclusive vehicle.

This line (AMBULANCE):
				<Item>
					<VehicleModels>
						<Vehicle>ambulance</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_M_Paramedic_01</Ped> 
					</PedModels>
				</Item>
This is the vehicle that will response when a pedestrian was injured on the streets. By default, the Ambulance is the only vehicle listed.

This line (FIRE_TRUCK):
				<Item>
					<VehicleModels>
						<Vehicle>firetruk</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Fireman_01</Ped> 
					</PedModels>
				</Item>
This is the vehicle that will response when a fire breaks out in any accessible parts of San Andreas. By default, the Fire Truck is the only vehicle listed.

This line (POLICE_ROAD_BLOCK_CARS):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
				</Item>
				<Item>
					<ZoneType>VEHICLE_RESPONSE_COUNTRYSIDE</ZoneType>
					<VehicleModels>
						<Vehicle>SHERIFF</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Sheriff_01</Ped> 
					</PedModels>
					<ObjectModels>
						<Object>prop_roadcone01a</Object>
					</ObjectModels>
				</Item>
				<Item>
					<VehicleModels>
						<Vehicle>police3</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
					<ObjectModels>
						<Object>prop_roadcone01a</Object>
					</ObjectModels>
				</Item>
These were the vehicles used as roadblocks when the player had a wanted level of 3 or more. Note that the vehicles listed in the VEHICLE_RESPONSE_ARMY_BASE group were the vehicles used when near Fort Zancudo, vehicles listed in
the VEHICLE_RESPONSE_COUNTRYSIDE group were the vehicles used in Blaine County area, and the one with no zone type were the vehicles used in Los Santos area.

This line (POLICE_ROAD_BLOCK_RIOT):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
				</Item>
				<Item>
					<IsMutuallyExclusive value="true" />
					<VehicleModels>
						<Vehicle>riot</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
					<ObjectModels>
						<Object>prop_roadcone01a</Object>
					</ObjectModels>
				</Item>

These were the vehicles used as tactical roadblocks that can appear at wanted level of 5 and rarely in wanted level 4. By default, the Police Riot were the only vehicles listed.

This line (POLICE_ROAD_BLOCK_SPIKE_STRIP):
				<Item>
					<ZoneType>VEHICLE_RESPONSE_ARMY_BASE</ZoneType>
				</Item>
				<Item>
					<IsMutuallyExclusive value="true" />
					<VehicleModels>
						<Vehicle>policet</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
					<ObjectModels>
						<Object>p_ld_stinger_s</Object>
					</ObjectModels>
				</Item>
These were the vehicles that appear in a spike strip roadblock. By default, the Police Transporter were the only vehicles listed.

This line (GANGS):
				<Item>
					<VehicleModels>
						<Vehicle>cavalcade</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>G_M_Y_PoloGoon_01</Ped>
					</PedModels>
				</Item>
These were the vehicles that were driven normally by gangs that can be found around South and East Los Santos.

This line (POLICE_BOAT):
				<Item>
					<VehicleModels>
						<Vehicle>predator</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_Cop_01</Ped> 
					</PedModels>
				</Item>
These were the vehicles that were used by cops during pursuit in water areas.

This line (ARMY):
				<Item>
					<VehicleModels>
						<Vehicle>mesa3</Vehicle>
					</VehicleModels>
					<PedModels>
						<Ped>S_M_Y_BlackOps_01</Ped>
					</PedModels>
				</Item>
These were the vehicles that were used by the military during pursuit. Unused by default, as army won't chase the player in max. wanted level.

Note:
Do not change ped models and object models in any of the line, it causes bugs!