Dedicating Managed Lanes
The managed lanes most familiar to motorists are dedicated to managed lane operation through a structural barrier, as in the case of the reversible lanes on the Kennedy Expressway and the express lanes on the Dan Ryan Expressway. Since these facilities are separated by barriers, separate shoulders are required for safety. In these cases, the managed lanes facility is very expensive, requiring both additional concrete construction and right-of-way. Sometimes, lower-cost alternatives are required by economic and engineering considerations. These low-capital options include separating managed lanes with a pavement buffer with appropriate markings and closely spaced, flexible plastic pylons.
In some cases of lane management strategies, e.g. left-lane truck prohibitions, no separation of lanes has been implemented. Typically, lane separation is required when the managed lane operates at a different speed than any unmanaged lanes. If managed lanes operate at substantially different speeds than regular lanes, lane changes to and from the managed lanes should be controlled with lane separations to reduce the safety hazard of vehicle conflicts at different speeds. Thus, a greater benefit for a managed lane facility in the form of higher managed lane speeds will require a more substantial investment.
How many lanes should be dedicated to a managed lane facility? Frequently, initial managed lane proposals are for a single lane in each direction, as in the initial proposal for a high-occupancy lane in the Eisenhower Expressway right-of-way. However, a single-lane facility has disadvantages in case of incidents, even the most minor of which may close the facility. An advantage of multiple-lane facilities is that in normal operations, having a two-lane facility will allow vehicles to pass each other, preventing blockages by slow-moving vehicles. Thus, if passenger vehicles are to be accommodated, a managed lane facility should be at least two lanes in each direction (if applicable). If a single lane is to be added to an existing facility to create a managed lane facility, facility planners should consider reallocating a general-purpose lane to the managed lane facility to maximize safety and traffic flow.
Taking the concept of managed lanes to a higher level, an entire freeway can be managed, particularly through the price mechanism. Managing an entire freeway has an advantage over managing a subset of lanes because it requires less right-of-way and requires less money for construction and enforcement. Safety is dramatically improved on a managed freeway because the speed differential inherent in most managed lane scenarios is eliminated – all lanes travel at the higher, uncongested speed. In addition, the clearing price for a managed freeway is lower than for individual lanes. However, such a facility reduces the choice of prices for travelers.
What, if any, existing highway facilities are good candidates for managed lanes? Why? What experience, if any, have you had with managed lanes facilities in other areas? Can you tell us about these facilities?