1. General. All the lane-use signals discussed in TFGAP-U are intended for long stretches, perhaps kilometers, of roadway. A left-turn arrow, for instance, regulates drivers turning left at any cross street or driveway in the controlled region. On urban surface streets there can be multiple intersections in the span between successive signals over a lane.

Altogether different are the widely-used lane-control (as opposed to lane-use) signs near intersections; one familiar example reads RIGHT LANE MUST TURN RIGHT while other examples use arrow symbols. The MUTCD describes the signs starting at section 2B.20 and the related pavement markings in section 3B.19. Important to remember is that these signs and markings pertain to motorists approaching one particular intersection, and their regulatory effect on a driver expires as soon as she clears that intersection.

Lane-control management is simpler than lane-use management because with lane control there is no concern with reversible lanes or the two-way left-turn lane.

2. Signals. In congested areas, lane control is sometimes handled not by signs but by special lights, roughly similar to lane-use signals, mounted over the center of each lane to be controlled. Signals are installed in order that lane control can be changed throughout the day. For example, a three-lane approach to some busy intersection might be configured as follows.

• Morning rush-hour drivers:
  • turn left from the left and center lanes,
  • go straight from the center and right lanes,
  • turn right from the right lane.
• Mid-day drivers:
  • turn left from the left lane,
  • go straight from any lane,
  • turn right from the right lane.
• Evening rush-hour drivers:
  • turn left from the left lane,
  • go straight from the left lane,
  • turn right from the center and right lanes.
• Overnight drivers:
  • same as mid-day

Because lane-control and lane-use signaling are different, they should use different symbols. Signals similar to the following are already in use in some areas. They mimic many of the standard signs used for this purpose, except that the signs most frequently have a black arrow on a white background.

These lane-control signals are black and white to provide ample contrast with the standard red-yellow-green signals that are likely installed to govern right of way at the intersection proper. Any intersection carrying enough traffic to justify changeable lane control in its approaches probably has enough volume to warrant a right-of-way signal, so it is important that motorists be able to tell the difference.

Additionally, lane-control management arrows differ from lane-use management arrows in color and shape, as discussed in TFGAP-U. Still, it may not be wise to use both categories of signal in the same section of highway (lane-use for general regulation and lane-control for special cases near intersections) because the combination could well have an interpretation too complicated for drivers in heavy traffic to sort out.

3. First Version of Display Sequence. A primitive attempt at signaling the schedule in the bulleted list above might show these lights over the three lanes:

Table One.
Step Number	Nature	Left Lane	Center Lane	Right Lane
1	Morning
2	Mid-day
3	Evening
4	Overnight
Return to step one.

It turns out that this simple plan has weaknesses. Some motorists will react more quickly than others to a change in signal; but when a light changes, even the most alert and prudent driver may have passed the point where she can redirect her vehicle in accordance with the limitations imposed by the new signal. That is why clearance intervals must be included in the sequencing of lane-control signals.

4. Implicit Clearance. Sometimes two lane-control movements cannot be used at the same time because of conflicts in vehicle paths: for instance, cars turning left from the center lane would collide with vehicles in the left lane going straight. While no planner would ever schedule these two movements simultaneously, there is still a danger if they occur in immediate succession. Because no clearance interval is included in the conversion from morning to mid-day in the table one, the following scenario could unfold, all within the space of a minute:

1. The traffic light at this intersection turns red.
2. Motorist "X", who plans to turn left, approaches the intersection a moment before the morning phase ends. Seeing that the left turn will be permitted from the center lane, she moves into that lane and waits at the stop bar for a green light.
3. The lane-control signal changes directly from the morning settings to the mid-day settings, without a clearance period. However, motorist "X", who is looking around the intersection, does not notice the change.
4. Motorist "Y", who plans to go straight, now approaches the intersection and moves into the left lane because she sees the mid-day signal permitting left-lane drivers to go straight. She, too, stops at the red light.
5. The light turns green and both drivers proceed. Motorist "X", who is turning left from the center lane, collides with motorist "Y", who is going straight in the left lane.

5. Second Version of Display Sequence. To remedy this issue is needed a more highly articulated schedule, with implicit clearance intervals, as displayed in table two. (Another kind of clearance, explicit, will be introduced later.)

Table Two.
Step Number	Nature	Left Lane	Center Lane	Right Lane	Comments
1	Morning				Add left turn to center lane (from last step).
	Implicit Clearance				Remove left turn from center lane.
2	Mid-day				Add straight movement to left lane.
	Implicit Clearance				Remove straight movement from right lane.
3	Evening				Change center lane from straight to right.
	Implicit Clearance				Change center lane from right to straight.
4	Overnight				Add straight movement to right lane.
	Implicit Clearance				Remove straight movement from left lane.
Return to step one.

An estimate is that the clearance interval should last through about three cycles of the red-yellow-green signals of the intersection proper, or approximately five minutes.

Implicit clearance is not necessary when some capacity of a lane is to be ended, and no conflicting movement from another lane is to begin.

While implicit clearance solves a safety problem, another matter remains as detailed in the next section.

6. Explicit Clearance. Suppose that step C of the five-step scenario above is changed, and motorist "X" does notice the changing of the lane-control signal, but only after she is stopped at the edge of the intersection. Through no fault of her own she is now in the wrong lane to complete her left turn, and her realization of what has happened may foster contempt for "unfair" traffic signals.

This problem can be addressed by adding explicit lane-control clearance signals, in the form of a flashing arrow, for a movement that will be prohibited in a few minutes. This tells a motorist already in the lane to continue, and it warns drivers who are still early in their approach that they should seek some other lane.

7. Third Version of Display Sequence. The fully developed schedule appears in the table below. Flashing signals can be implemented with two alternating images, although animations of greater sophistication are possible.

Table Three.
Step Number	Nature	Left Lane	Center Lane	Right Lane	Comments
1	Morning				Add left turn to center lane (from step 4b).
1a	Explicit Clearance		Alternating		Remove left turn from center lane.
1b	Implicit Clearance
2	Mid-day				Add straight movement to left lane.
2a	Explicit Clearance			Alternating	Remove straight movement from right lane.
2b	Implicit Clearance
2c	Conversion				Add right turn to center lane.
2d	Explicit Clearance		Alternating		Remove straight movement from center lane.
3	Evening				If the right turn were also being added to the left lane, this step would be preceded by an implicit phase for clearance of the straight movement in the center lane.
3a	Conversion				Add straight movement to center lane.
3b	Explicit Clearance		Alternating		Remove right turn from center lane.
3c	Implicit Clearance
4	Overnight				Add straight movement to right lane.
4a	Explicit Clearance	Alternating			Remove straight movement from left lane.
4b	Implicit Clearance
Return to step one.

While this plan may seem complicated, the reader should bear in mind that at any moment motorists will see only those signals that they need to see -- in other words exactly one row of the table. Hence only the engineers need worry about the volume of detail.

Here is a brief summary of the difference between the two types of clearance phases.