Law Enforcement

Law Enforcement is basically describing what is left on civilian police forces during the Infection. Like the Military, they played a part in attempting to contain the Infection and evacuate citizens during the early days of the infection. Also like the Military, many of its members have succumbed to the Infection and so appear (in their uniforms) as part of the huge army of Common Infected (though only in the original Left 4 Dead).

Involvement with containing The Infection
In Fairfield, police were working with the military and CEDA in an attempt to fight and stop the Green Flu epidemic from spreading after things got out of hand. At first, CEDA policy banned civilians from possessing firearms, which the police likely enforced and may have just arrested Infected individuals attacking humans and people taking advantage of the situation such as looters ,people taking the law into their own hands (such as killing infected back when people thought they were still human and could be saved), and even Francis while SWAT handled large groups of infected with non lethal tatics such as tears gas and riot shields at first but when the situation got worse, police possibly handed out weapons to civilians and began to use lethal force as they lost more members to the sickness and out of desperation. As the infection was spreading, law enforcement in other cities began to help evacuation efforts by keeping the public orderly and calm and was likely helped out by security officers hired by CEDA; this can be seen in such areas like New Orleans and Fort Harris County.It is interesting to note that despite SWAT weapons being available no other traces (such as vehicles,corpses,etc) of SWAT are found in ethier two Left 4 Dead games which is odd since SWAT would have to be called in to control mobs of infected using riot control tactics or as firing squads to eliminate large groups of infected as the situation was deteriorating.

Technologies used in the series
Police Cars ― These are a common form of transport for police, and through most campaigns in both games can be mostly found in evacuation areas but are also found in locations such as highways or in the streets occasionally with their light bars on. Their design is based on the common American and Canadian patrol car, the Ford Crown Victoria.

It being the only police vehicle seen so far is quite odd as police departments use an array of specialty vehicles such as helicopters, watercraft, command post, vans, trucks, all terrain vehicles, motorcycles, and SWAT armored vehicles. A plausible reason is that they have been destroyed by the Infected or may have been abandoned.

Also many weapons used by police and SWAT can be seen:

Left 4 Dead
 * Auto Shotgun
 * Assault Rifle
 * M1911 Pistol
 * Pump Shotgun
 * Submachine Gun

Left 4 Dead 2
 * Glock
 * Combat Shotgun
 * Sniper Rifle
 * Nightstick
 * MP5
 * Steyr Scout
 * Accuracy International AWSM