Traffic Control Design · Study · PE Civil: Transportation · FE → PE Prep
Traffic Control Design
6% of exam
Permanent signs and pavement markings, sign placement and legibility, and temporary traffic control: work-zone taper lengths, buffer space, and channelizing-device spacing.
4 concepts
A. Permanent signs and pavement markings
Permanent Signs & Pavement Markings
The regulatory/warning/guide sign families and their shapes and colors, the legibility basis for sizing and placing signs, and the color-and-pattern logic of longitudinal pavement markings.
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Markings, Delineation & Sign Supports
No-passing-zone and lane-use markings keyed to passing sight distance, raised pavement markers and delineators with their spacing and color rules, and breakaway/yielding sign supports.
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B. Temporary traffic control
Work-Zone Taper Lengths & TTC
How to size a merging taper from the MUTCD L = WS²/60 and L = WS rules, set the shifting/shoulder ratios, space channelizing devices, and lay out the four temporary-traffic-control areas.
Work zones are where a routine highway becomes a hazard, and the temporary-traffic-control (TTC) plan is the engineering that keeps that hazard survivable: it tells the driver, well in advance, what is changing, and then moves traffic out of the way of the workers smoothly enough that nobody has to brake hard or swerve. On the PE Civil Transportation exam this is the most reliably tested corner of the traffic-control knowledge area, and it is almost pure recall-plus-arithmetic: a merging-taper length, a couple of taper ratios, a device spacing, and the names of the four areas. No taper method lives in the NCEES PE Civil Reference Handbook — its construction chapter touches work-zone safety only in passing (OSHA-style noise/exposure notes), with no taper, spacing, or buffer formula. The governing source is the MUTCD, Part 6 (Temporary Traffic Control), and that is what this concept teaches and cites.
The four areas of a TTC zone
Every lane-closure setup, read in the direction of travel, is built from four areas in a fixed order. First the driver passes the advance warning area, a series of signs that tell what to expect. Then comes the transition area, which contains the taper that physically redirects traffic out of the closed lane. Next is the activity area — the part containing the work space (closed to traffic and usually fenced or barricaded), a longitudinal buffer space upstream of the workers, and the traffic space that vehicles actually use. Finally the termination area returns traffic to its normal path, often with a short downstream taper and an END ROAD WORK sign. Get the order and the contents of these four right and most layout questions answer themselves.
The transition area: merging-taper length
The taper is the heart of the design. Its required length depends on how far you are shifting traffic (W, the offset width in feet, usually one lane ≈ 12ft) and the speed (S, the posted speed, off-peak 85th-percentile speed, or anticipated operating speed, in mph). The MUTCD splits the rule at 45mph because the underlying driver-path model changes from a low-speed lateral-shift relationship to a high-speed one. Below the break the length grows with the square of speed; at and above it the relationship is linear:
L=⎩⎨⎧60WS2WSS≤40mphS≥45mph
Other taper types and their ratios
Not every taper is a full merging taper, and the exam loves to test the ratios. A shifting taper, which slides traffic laterally without dropping a lane, needs only about half the merging length, ≥0.5L. A shoulder taper (closing a shoulder) needs about a third, ≥31L. A one-lane, two-way taper used to alternate traffic past a short closure is short and blunt, roughly 50
Spacing the channelizing devices
Cones, drums, and tubular markers form the taper, and their spacing is governed by a simple speed rule: within a taper the longitudinal spacing in feet should not exceed the speed limit in mph (so a 60mph taper uses devices no more than 60ft apart). Along tangent sections of the activity area the spacing may double, up to about twice the speed in feet. To count the devices in a taper, divide the taper length by the spacing and add one for the device at the upstream end.
staper≤S(ft),stangent≤2S(ft),N=sL+1
The buffer space
Between the downstream end of the taper and the first worker there should be a longitudinal buffer space, a deliberately empty cushion containing no equipment and no workers. Its purpose is recovery distance: a vehicle that fails to merge has room to stop before reaching anyone. The MUTCD sizes the longitudinal buffer using stopping-sight-distance values keyed to speed, so it grows quickly with speed — on the order of 570ft at 60mph. There is also a lateral buffer (the offset between the traffic space and the work space). Treat the buffer as non-negotiable: it is the single feature that turns a missed merge into a near-miss instead of a fatality.
Read the speed first and pick the correct branch — WS2/60 at ≤40mph, WS at ≥45mph
Key equations
Merging taper, low speedL=60WS2
Minimum merging-taper length (ft) for S≤40mph
Worked examples
Freeway lane-closure taper
Problem. A right-lane closure is needed on a 60mph freeway. The lane being closed is 12ft wide. Find the minimum merging-taper length, the corresponding shifting- and shoulder-taper lengths, and the number of drums in the merging taper at the maximum allowed spacing.
Solution. Because S=60mph≥45
Common pitfalls
•Using L=WS at low speed (or WS2/60 at high speed). The break is at 45mph: square-law at ≤40
References
Manual on Uniform Traffic Control Devices (MUTCD), Part 6 — Temporary Traffic Control (taper lengths, channelizing-device spacing, buffer space)
AASHTO, A Policy on Geometric Design of Highways and Streets (Green Book, GDHS-7) — stopping sight distance used to size the longitudinal buffer
NCEES PE Civil Reference Handbook — Construction §2.6.2 Work Zone and Public Safety (context only; contains no taper method)
Temporary Traffic Control Zones & Devices
The advance-warning area and its sign spacing, channelizing-device spacing in tapers and tangents, flagging and the flagger station, and how the activity area is assembled.
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to
100ft
. The optional downstream taper in the termination area is about
100ft
per lane. Remember that the merging taper is the longest of the family because it is doing the most work — forcing a complete lane change.
— mixing them is the most common error. Default to
W=12ft
for one full lane unless a width is given. Once you have
L
, the rest cascades: shifting
=0.5L
, shoulder
=L/3
, device spacing
=S
feet in the taper, count
=L/s+1
. Keep the four areas straight (advance → transition → activity → termination) and remember the buffer space sits inside the activity area, upstream of the workers. Finally, never cite the NCEES handbook for any of these numbers — they are MUTCD Part 6, and a question that asks for the source wants 'MUTCD'.
.
W
= lateral offset (ft),
S
= speed (mph).
Merging taper, high speedL=WS
Minimum merging-taper length (ft) for S≥45mph.
Shifting taperLshift≥0.5L
Lateral-shift taper (no lane drop) is at least half the merging length.
Shoulder taperLshoulder≥31L
Closing a shoulder needs about one-third of the merging length.
One-lane two-way taper50ft≤L1L2W≤100ft
Short blunt taper where one lane serves both directions alternately.
Device spacing (taper)staper≤S(ft)
Channelizing-device spacing in a taper, in feet, not to exceed the speed in mph.
Device spacing (tangent)stangent≤2S(ft)
Along tangent activity-area sections spacing may roughly double.
Device count in a taperN=sL+1
Number of channelizing devices over a taper of length L at spacing s (add one for the upstream device).
Longitudinal buffer spaceLbuffer≈SSD(S)
Empty recovery cushion ahead of the work space, sized to stopping sight distance.
, use the high-speed rule:
L=WS=12×60=720ft
.
Shifting taper
=0.5L=0.5(720)=360ft
. Shoulder taper
=L/3=720/3=240ft
.
Maximum device spacing in the taper
=S=60ft
. Number of drums
N=L/s+1=720/60+1=12+1=13
drums.
Sanity: at
60mph
the linear rule gives a long taper (
≈0.14mi
), as expected for a full lane shift at high speed; the
13
drums at
60ft
span exactly
720ft
. ✓
L=WS=12(60)=720ft,N=60720+1=13
Low-speed urban taper
Problem. On a 35mph urban arterial an 11ft lane is being closed. Find the minimum merging-taper length and the number of cones at the maximum spacing.
Solution. Because S=35mph≤40, use the low-speed rule: L=60WS2=6011(35)2=6011(1225)=6013,475=224.6ft, round up to 225ft.
Maximum spacing =S=35ft. Cones N=L/s+1=225/35+1=6.4+1→8 cones (round the device count up).
Sanity: had you wrongly used L=WS=11(35)=385ft you would over-build by 70% — a clear sign you took the wrong branch. The squared rule correctly yields the shorter low-speed taper. ✓
L=60WS2=6011(35)2≈225ft
Shifting taper and buffer space at 50 mph
Problem. A 50mph highway needs a lateral shift of one 12ft lane (a shifting taper, no lane drop) around an off-shoulder excavation, followed by a longitudinal buffer ahead of the work. Find the shifting-taper length and the buffer length, given that the buffer is sized to the stopping sight distance at 50mph (use tr=2.5s and a deceleration of 11.2ft/s2).
Solution. First size the reference merging taper at S=50≥45: L=WS=12×50=600ft.
A shifting taper is at least half of that:
Lshift=0.5(WS)=300ft,SSD50≈424ft
, linear at
≥45
. Check the speed before you pick the formula.
•Forgetting the units convention: W is in feet, S in mph, and L comes out in feet. The 60 in the denominator is a dimensional constant baked into the MUTCD rule, not a unit conversion you can change.
•Confusing the taper ratios: the shifting taper is 0.5L and the shoulder taper is L/3 — not the reverse. The merging taper is always the longest.
•Citing the NCEES PE Civil Reference Handbook for taper length or device spacing. These methods are MUTCD Part 6; the handbook contains no taper formula.
•Dropping the '+1' when counting devices, or counting buffer-space devices — the buffer space is intentionally empty of devices, equipment, and workers.
•Putting workers in the buffer space. The buffer is a recovery cushion; the work space begins downstream of it.
•Treating the four areas as optional or reordering them. In the direction of travel it is always advance warning → transition → activity → termination.