Temporary Signal Versus Flaggers for a Bridge-Widening Lane Closure
A bridge-widening project on a two-lane rural road (posted 45 mph, 1,800 vpd, 8 percent trucks) requires closing one lane for three months, 7 a.m. to 5 p.m., Monday through Friday. The one-lane section is 480 ft long between the portals. Stopping sight distance from either end to the other is about 300 ft because of a horizontal curve at the north portal. The contractor proposes a portable two-head temporary traffic signal at each end on a fixed 90 s cycle, powered by a generator, with a single ROAD WORK AHEAD sign at 500 ft on each approach. There is no pedestrian facility, but a school bus stop is 200 ft south of the work zone on the open lane's side, and the bus stops at 7:15 a.m. and 3:45 p.m.
1. Is the proposed temporary signal acceptable, and what deficiencies must be corrected?
Read the worked answer
The signal type is defensible; the setup as proposed is not.
Sight distance. At 45 mph, stopping sight distance is about 1.47 x 45 x 2.5 + 45 squared / (30 x 0.35) = 165 + 193 = 358 ft. The available 300 ft to the curve is short of that, so a signal alone does not fix the approach. Require advance warning with proper spacing, a ONE LANE ROAD AHEAD sign with a distance plaque, and a flagger station ready to take over if the signal fails. Consider including a buffer space ahead of the signal.
Lens size. At a posted speed over 40 mph the through-movement signal indications must be 12 in., with backplates and retroreflective borders.
Fail-safe. On power or controller failure, both approaches must flash red and stop signs must be posted on both approaches. A generator alone is not a fail-safe; require backup power sized for the shift plus a fueling plan.
Visibility. Each signal face must be visible for the stopping distance, which means placement relative to the curve matters more than the sign spacing.
2. Check the timing concept. What yellow and red clearance does this one-lane section need, and is a fixed 90 s cycle sensible?
Read the worked answer
Yellow: 1.0 + 66/20 = 4.3 s. A driver must not be able to enter on yellow and meet opposing traffic.
Red clearance: the time to travel the full one-lane section at design speed = 480/66 = 7.3 s, plus the vehicle length. Use about 7.5 to 8 s, not a nominal 2 or 3 s. This is the interval that prevents a head-on collision, so it is the single most important setting in the plan.
Green: with 45 s per direction, green is 45 - 4.3 - 7.5 = about 33 s. At 1,800 vpd with a peak hour perhaps 15 percent of daily traffic, arrivals per 45 s are only 2 to 3 vehicles, so capacity is not the constraint.
Fixed versus actuated: a fixed 90 s cycle is acceptable for a temporary installation but imposes up to 45 s of unnecessary delay on a low-volume road. If the equipment supports it, use presence detection with a maximum green and a rest-in-red with detection, plus a maximum green to protect against a stuck call. Do not shorten the clearance intervals to shorten the cycle.
3. What temporary traffic control and coordination issues remain, including the school bus stop?
Read the worked answer
School bus stop. The bus must be able to stop and re-enter the open lane safely. The stop is 200 ft south of the zone, so the bus will stop inside the approach to the signal. Verify the bus stops outside the taper and buffer, coordinate with the school district, and confirm the 7:15 a.m. and 3:45 p.m. stops fall inside the 7 a.m. to 5 p.m. work window. If the bus cannot safely stop, relocate the stop for the duration and notify parents, or suspend the lane closure during those windows.
TTC zone completeness. Advance warning area at the correct spacing for 45 mph, a proper taper and buffer space, channelizing devices with reflectors, and the drum or barrier line held at the correct offset. The work zone must not narrow the open lane below the usable width needed for trucks at 8 percent of traffic.
Night and weekend condition. Confirm what remains in place when the crew leaves; the one-lane section cannot be left uncontrolled.
Special considerations. Verify the curve does not hide the signal from the stopping distance, as noted, and keep a flagger on call. Document the setup and the engineer's approval.