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

Exam areas

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.

Unlocks with an access pass — one-time payment, no auto-renew. View passes

Pass holders

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.

Unlocks with an access pass — one-time payment, no auto-renew. View passes

Pass holders

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 ≈

12\,\text{ft}

) and the speed (

S

, the posted speed, off-peak 85th-percentile speed, or anticipated operating speed, in mph). The MUTCD splits the rule at

45\,\text{mph}

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 = \begin{cases} \dfrac{W S^{2}}{60} & S \le 40\ \text{mph} \\[2mm] W S & S \ge 45\ \text{mph} \end{cases}

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,

\ge 0.5L

. A shoulder taper (closing a shoulder) needs about a third,

\ge \tfrac{1}{3}L

. A one-lane, two-way taper used to alternate traffic past a short closure is short and blunt, roughly

50

100\,\text{ft}

. The optional downstream taper in the termination area is about

100\,\text{ft}

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.

L_{\text{shift}} \ge 0.5L, \qquad L_{\text{shoulder}} \ge \tfrac{1}{3}L, \qquad L_{\text{downstream}} \approx 100\ \text{ft/lane}

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

60\,\text{mph}

taper uses devices no more than

60\,\text{ft}

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.

s_{\text{taper}} \le S\ (\text{ft}), \qquad s_{\text{tangent}} \le 2S\ (\text{ft}), \qquad N = \frac{L}{s} + 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. When a longitudinal buffer is used, MUTCD Table 6C-2 offers stopping-sight-distance values keyed to speed for its length, so it grows quickly with speed — on the order of

570\,\text{ft}

60\,\text{mph}

. There is also a lateral buffer (the offset between the traffic space and the work space). The MUTCD makes the longitudinal buffer an option, but treat it as essential: it is the single feature that turns a missed merge into a near-miss instead of a fatality.

L_{\text{buffer}} \approx \text{SSD}(S) \quad \text{(MUTCD stopping-sight-distance basis)}

Exam strategy

Read the speed first and pick the correct branch —

WS^{2}/60

\le 40\,\text{mph}

WS

\ge 45\,\text{mph}

— mixing them is the most common error. Default to

W = 12\,\text{ft}

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'.

Key equations

Merging taper, low speed

L = \dfrac{W S^{2}}{60}

Minimum merging-taper length (ft) for $S \le 40\,\text{mph}$ . $W$ = lateral offset (ft), $S$ = speed (mph).

Merging taper, high speed

L = W S

Minimum merging-taper length (ft) for $S \ge 45\,\text{mph}$ .

Shifting taper

L_{\text{shift}} \ge 0.5\,L

Lateral-shift taper (no lane drop) is at least half the merging length.

Shoulder taper

L_{\text{shoulder}} \ge \tfrac{1}{3}\,L

Closing a shoulder needs about one-third of the merging length.

One-lane two-way taper

50\ \text{ft} \le L_{\text{1L2W}} \le 100\ \text{ft}

Short blunt taper where one lane serves both directions alternately.

Device spacing (taper)

s_{\text{taper}} \le S\ (\text{ft})

Channelizing-device spacing in a taper, in feet, not to exceed the speed in mph.

Device spacing (tangent)

s_{\text{tangent}} \le 2S\ (\text{ft})

Along tangent activity-area sections spacing may roughly double.

Device count in a taper

N = \dfrac{L}{s} + 1

Number of channelizing devices over a taper of length $L$ at spacing $s$ (add one for the upstream device).

Longitudinal buffer space

L_{\text{buffer}} \approx \text{SSD}(S)

Empty recovery cushion ahead of the work space, sized to stopping sight distance.

Worked examples

Freeway lane-closure taper

Problem. A right-lane closure is needed on a $60\,\text{mph}$ freeway. The lane being closed is $12\,\text{ft}$ 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 = 60\,\text{mph} \ge 45

, use the high-speed rule:

L = WS = 12 \times 60 = 720\,\text{ft}

. Shifting taper

= 0.5L = 0.5(720) = 360\,\text{ft}

. Shoulder taper

= L/3 = 720/3 = 240\,\text{ft}

. Maximum device spacing in the taper

= S = 60\,\text{ft}

. Number of drums

N = L/s + 1 = 720/60 + 1 = 12 + 1 = 13

drums. Sanity: at

60\,\text{mph}

the linear rule gives a long taper (

\approx 0.14\,\text{mi}

), as expected for a full lane shift at high speed; the

13

drums at

60\,\text{ft}

span exactly

720\,\text{ft}

. ✓

L = WS = 12(60) = 720\ \text{ft}, \quad N = \tfrac{720}{60}+1 = 13

Low-speed urban taper

Problem. On a $35\,\text{mph}$ urban arterial an $11\,\text{ft}$ lane is being closed. Find the minimum merging-taper length and the number of cones at the maximum spacing.

Solution. Because

S = 35\,\text{mph} \le 40

, use the low-speed rule:

L = \dfrac{WS^{2}}{60} = \dfrac{11(35)^{2}}{60} = \dfrac{11(1225)}{60} = \dfrac{13{,}475}{60} = 224.6\,\text{ft}

, round up to

225\,\text{ft}

. Maximum spacing

= S = 35\,\text{ft}

. Cones

N = L/s + 1 = 225/35 + 1 = 6.4 + 1 \to 8

cones (round the device count up). Sanity: had you wrongly used

L = WS = 11(35) = 385\,\text{ft}

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 = \frac{W S^{2}}{60} = \frac{11(35)^{2}}{60} \approx 225\ \text{ft}

Shifting taper and buffer space at 50 mph

Problem. A $50\,\text{mph}$ highway needs a lateral shift of one $12\,\text{ft}$ 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 $50\,\text{mph}$ (use $t_r = 2.5\,\text{s}$ and a deceleration of $11.2\,\text{ft/s}^2$ ).

Solution. First size the reference merging taper at

S = 50 \ge 45

L = WS = 12 \times 50 = 600\,\text{ft}

. A shifting taper is at least half of that:

L_{\text{shift}} = 0.5L = 0.5(600) = 300\,\text{ft}

. Buffer length from stopping sight distance with

v = 50\,\text{mph}

\text{SSD} = 1.47 v t_r + \dfrac{1.075\,v^{2}}{a} = 1.47(50)(2.5) + \dfrac{1.075(50)^{2}}{11.2} = 183.75 + 240.0 = 424\,\text{ft}

, so the longitudinal buffer is about

425\,\text{ft}

. Sanity: the shifting taper (

300\,\text{ft}

) is shorter than a full merging taper (

600\,\text{ft}

) because it does less work, and the

\approx 425\,\text{ft}

buffer sits between

\text{SSD}

45\,\text{mph}

(

\approx 360\,\text{ft}

) and

60\,\text{mph}

(

\approx 570\,\text{ft}

), as expected. ✓

L_{\text{shift}} = 0.5(WS) = 300\ \text{ft}, \quad \text{SSD}_{50} \approx 424\ \text{ft}

Common pitfalls

•Using $L = WS$ at low speed (or $WS^{2}/60$ at high speed). The break is at $45\,\text{mph}$ : square-law at $\le 40$ , linear at $\ge 45$ . 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.

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.

Unlocks with an access pass — one-time payment, no auto-renew. View passes

Pass holders