There is no certain way of reducing traffic demand - it is often a diverse and difficult issue. Various methods need to be combined in response to local requirements. Even in the simplest method of information provision, consultation with and coordination of the parties involved is essential, with careful monitoring of the effects of controls. This is particularly true for direct methods of demand management by means of access control.
A common method is to remove through traffic from a controlled zone by restricting entrance to that zone only to users who have an electronic pass, or through vehicle licence-plate recognition. (See Policing / Enforcement) This is typically done by signs that announce the restrictions and a gate or barrier at each entrance of the zone where the electronic pass or vehicle licence plate can be checked. Checking is sometimes done manually although there are ways to do this automatically through the use of ANPR and "permitted lists" (lists of authorised vehicles), or with on-board transponders as a basis for enforcement.
Entry restrictions are always politically difficult to implement, although perhaps not as difficult as road pricing. The issues commonly associated with this restriction are political, the cost of equipment or staff required to enforce the restriction and the numbers of users who seek exemption or try to evade the system. In the case of a High Occupancy Vehicle (HOV) lane, for example, there have been cases where people are paid to ride in the user’s vehicle. Also, the restriction of traffic may have negative effects on the area’s economy, which may be a source of criticism. The challenge always is to reduce vehicular traffic without limiting productive activities within the area.
Ramp metering is a method to restrict the entry of vehicles to a motorway from a selected on-ramp in order to maintain free-flow conditions on the main carriageway. The implementation is fairly easy, since the number of ramps is limited and they can be controlled by the network operator. Ramp metering and entry restriction are widely used in the USA, either based on the numbers of vehicles, or selectively (for example High Occupancy Vehicles (HOV) only). (See Highway Traffic Management)
Ramp metering is becoming increasingly common outside USA. Implementation issues are not large, although there can be resistance from some users. An issue which sometimes needs careful attention is the storage capacity of the ramps, since queuing which tails back onto surrounding main roads may cause congestion to spread and reduce the level of service on the arterial roads.