A common important element of a TTC zone is a roadway taper. Tapers may be used in both the transition and termination areas. Tapers are created using a series of channelizing devices and/or pavement markings placed to move traffic out of or into its normal path. The standard diagrams in this handbook show tapers of adequate length for most urban conditions. Whenever tapers are to be used near interchange ramps, crossroads, curves, or other influencing factors, it may be desirable to adjust the length of the tapers. Longer tapers are not necessarily better than shorter tapers (particularly in urban areas characterized by short block lengths, frequent driveways, etc.), because extended tapers tend to encourage sluggish operation and to encourage drivers to delay lane changes The real test of taper length involves observation of driver performance after traffic controls are in place.
(See Figure 3 - Tapers and Buffer Space)
There are five different types of tapers, each of which is described below.
A. MERGING TAPER
The taper should be long enough to enable merging drivers to adjust their speeds and merge into a single lane before the end of the transition. The appropriate series of lane reduction/merge signs shall be erected in the advance warning area before a merging taper. The "ONE LANE ROAD AHEAD" sign shall not be used in lieu of the proper lane reduction/merge signs.
B. SHIFTING TAPER
A shifting taper is used when a lateral shift is needed to move traffic out of or into the normal path. Where more space is available, it may be beneficial to use longer-than-minimum distances. Guidance for changes in alignment may also be accomplished by using horizontal curves designed for normal roadway speeds. If a shifting taper is shorter than the minimum length for the roadway speed, then appropriate curve or turn warning signs with the appropriate advisory speed plates shall be erected in the advance warning area. A shifting taper be approximately half of the normal merging taper for the same speed.
C. SHOULDER TAPER
When work is occurring on the paved shoulder of a high-speed roadway, a shoulder taper may be beneficial. If used, shoulder tapers approaching the activity area should have a length of about one-third of a normal merging taper for the same speed. If a shoulder is used as a travel lane, either through practice or during a temporary traffic activity, a normal merging taper and proper advance warning signing should be used.
D. DOWNSTREAM TAPER
The downstream taper may be useful in termination areas to provide a visual clue to the driver that access is available to the original lane/path that was closed. When a downstream taper is used, it should have a minimum length of 100 feet per lane, with devices spaced about 20 feet apart.
E. ONE-LANE, TWO-WAY TAPER
The one-lane, two-way taper is used in advance of an activity area that requires the traffic space to be used alternately by traffic flowing in opposing directions. It shall serve the traffic direction that must shift sides. Typically, a flagger controls traffic. A short taper having a maximum length of 100 feet with channelizing devices at approximately 20-foot spacing should be used to guide traffic into the one-lane, two-way section. A "ONE LANE ROAD AHEAD" sign shall be placed in each direction in advance of a one-lane, two-way taper.