Work Zone - 3.01 Travel Time

3.01 Travel Time

The travel time delay should be estimated for the work zone to determine project significance as defined in Section 2.01 Project Significance. Consideration should be given to:

• Speed delay: Results from the reduced travel speed in the work zone, in comparison to the existing condition. This is affected by the temporary speed limit and by the capacity of the work zone.
• Queue delay: Additional time vehicles spend waiting in a queue. This is a result of the accumulation of vehicles upstream of a capacity reduction (i.e. a lane closure) when the demand exceeds the capacity.
• Control delay: Interruptions in traffic flow caused by temporary traffic control devices.
• Interruptions occur on all roadway types and include planned events (i.e. traffic regulators, temporary signals, temporary traffic stoppages).

Past experience should also be a factor when estimating travel times. Diversion rate of past projects in similar locations should be utilized to refine your estimation of work zone delay.

3.01.01 Uninterrupted Flow Facilities

Computing travel time for uninterrupted flow facilities (i.e. freeways or non-freeway without stop controlled or signal controlled intersections) consists of determining the speed delay and the queue delay.

A. Capacity Determination – Freeway

Table 3-1 Base Mainline Capacity and Table 3-2 Capacity Adjustment Factors should be used to estimate the capacity for each open lane. Apply factors known to adjust the base work zone capacity.

To begin the development of traffic control options, determine the number of lanes needed to handle the expected demand. An early check of available roadway capacity is critical as the vast majority of additional delay due to a work zone occurs when demand exceeds capacity for any appreciable length of time. The Highway Capacity Manual (HCM) Equation 10-9 can be used to determine the resulting adjusted capacity of the roadway lane:

C_a = {[(C_b + I_g + I_wt + I_wa ) × f_hv × f_lw × f_sc ] × N} − R

Where:
C_a = adjusted mainline capacity - vehicles per hour (vph)
C_b = base mainline capacity - vehicles per hour per lane (vphpl)
I_g = adjustment factor for geometrics (ranges from -150 to + 250 vphpl, could be zero). Select multiple factors that apply depending on the conditons; apply the cumulative adjustment factor.
I_wt = adjust factor for intensity of the work activity (ranges from -200 to +200 vphpl, could be zero). Select one factor
I_wa = adjustment factor for location of the work activity (ranges from -150 to +200 vphpl). Select one factor.
f_hv = adjustment for heavy vehicles as defined in HCM Equation 10-8. Values have been provided for typical truck volume ranges
f_lw = adjustment for lane/shoulder widths
f_sc = adjustment for lane/shoulder side clearance
N = number of lanes open through the work zone
R = manual adjustment for entrance ramps (vph)
The adjustment factor for work zone geometrics may be considered cumulatively. For example, work zone geometry when traffic uses a crossover and requires a traffic shift >3 feet would apply the cumulative effect of each condition as a part of the adjustment factor.

When determining the factors engineering judgement should be used to determine if the factors should be applied cumulatively. The base mainline capacity (C_b) and work zone capacity adjustment factors are provided in the tables below.

Table 3-1: Base Mainline Capacity

Number of Lanes		Starting Capacity per Lane
Normal	Open	Cb
3	1	1400
2	1	1550
5	2	1600
4	2	1700
3	2	1700
4	3	1750

Table 3-2: Capacity Adjustment Factors

* When calculating C_a the final result should range from 1,100 vphpl to 2,000 vphpl. If the result comes back outside of this range please contact MDOT's Work Zone Delivery Engineer for approval.
** If multiple work zone conditions occur, use engineering judgement to determine which factors to use. The final number must fall within th erange of the largest values for that adjustment factor.
*** Please send verified delay measurements to the WZMU. This will allow for adjustments to the numbers based on real world experiences.

Work Zone Conditions**	Capacity Adjustment Factors*
Geometry: traffic uses crossover	Ig	Subtract	150 vphpl
Geometry: traffic driving on shoulder		Subtract	150 vphpl
Geometry: traffic shift (> 3 feet)		Subtract	100 vphpl
Geometry: work activity adjacent, protected with temporary barrier		Add	250 vphpl
Geometry: work activity adjacent (< 15 feet), protected by channelizing devices		Subtract	100 vphpl
Work Type: pavement repairs (concrete or HMA)	Iwt	Subtract	200 vphpl
Work Type: rubblizing, reconstruction		Subtract	150 vphpl
Work Type: overhead activity (i.e. bridge work – painting, patching)		Subtract	150 vphpl
Work Type: cold milling, paving (concrete or HMA)		Subtract	100 vphpl
Work Type: guardrail or barrier		Add	50 vphpl
Work Type: signing installation		Add	50 vphpl
Work Type: occurs > 20 feet away		Add	100 vphpl
Work Activity: adjacent lane distance < 12 feet	Iwa	Subtract	100 vphpl
Work Activity: adjacent lane distance > 12 feet, < 20 feet		Subtract	50 vphpl
Work Activity: adjacent lane distance > 20 feet away		Add	150 vphpl
Work Activity: separated by median		Add	50 vphpl
12' lane width	flw	Multiply	1.00
11' lane width		Multiply	0.95
10' lane width		Multiply	0.90
Restricted one side	fsc	Multiply	0.95
Restricted both sides		Multiply	0.90
Trucks 0% to ≤ 5%	fhv	Multiply	1.00
Trucks > 5% to ≤ 10%		Multiply	0.98
Trucks > 10% to ≤ 15%		Multiply	0.95
Trucks > 15% to ≤ 20%		Multiply	0.93
Trucks > 20%		Multiply	0.90
Entrance ramp within 1,500 feet downstream from the end of the lane closure taper	R	Subtract	Hourly ramp volume (max 600 vph)

Example 3.1: Freeway

A construction project is planned for a two-lane freeway portion of US-127. The project will last for several months and involves complete reconstruction of the roadway and several bridges on each bound. One lane of traffic is proposed to be maintained in each direction by using temporary crossovers. Maintained traffic must drive on the shoulder. Lane widths will be reduced to 11 feet, with temporary concrete barrier separating opposing traffic. The traffic characteristics consist of 7% trucks. Calculate Lane capacity.

Solution 3.1: Freeway

The lane capacity is computed by starting with the base capacity (C_b) for a 2 lane freeway to a 1 lane freeway 1550 vphpl. The adjustment factor applied for geometry includes use of a crossover, with a > 3 foot shift, and traffic driving on the shoulder. Since we are only allowed to use one factor I_g = -100 (could range from 100 to 150). Although the work type is reconstruction, it is not in close proximity to the maintained traffic since it occurs on the closed bound of the freeway (I_wt = 100). The adjustment factors applied for work activity (I_wa = 50), lane width (f_lw = 0.95), side clearance (f_sc = 0.95), and percent trucks (f_hv = 0.98). The adjusted capacity is:

C_a = [(1550 − 100 + 100 + 50) × 0.95 × 0.95 × 0.98] = 1415 vph

Example 3.2: Freeway

A construction project is planned for an urban three lane freeway portion of I-696. The project will last a few months and involves replacement of median barrier and lighting fixtures. The median lane will be closed in each direction during some parts of the day using channelizing devices. The work activity is less than 12 feet from traffic. The traffic characteristics consist of 15% trucks. Calculate lane capacity.

Solution 3.2: Freeway

The lane capacity is computed by starting with the base capacity (C_b) for a 3 lane freeway to a 2 lane freeway 1700 vphpl. Adjustments are applied for close proximity intense work activity (I_g = -100), (I_wt = 50), (I_wa = -150), side clearance (f_sc = 0.95), percentage trucks (f_hv = 0.95). The adjusted capacity for two open lanes is:

C_a = {[(1700 − 100 + 50 −1 00) × 0.95 × 0.95] ×2} = 2798 vph or 1399 vphpl

B. Traffic Demand Determination

An estimation of the hourly traffic demand is necessary to proceed with travel time delay and queue estimations. Existing traffic volume data is regularly collected on most state trunklines. Information regarding traffic data is provided in Section 2.03.01.A Traffic Information.

The level of detail of traffic volume data depends on the level of analysis. Hourly traffic volume data is necessary to accurately evaluate mobility impacts on facilities that may have fluctuations in traffic demand throughout the day. This is of particular importance if the temporary traffic control plan considers implementing traffic restrictions or lane closures only during certain portions of the day.

Weekend and weekday volumes are necessary when determining week day restrictions. Reviewing traffic volumes from the same time frame is important to make sure the traffic patterns are similar. This is amplified when dealing with an area that has a yearly traffic generator such as a school or seasonal traffic.

C. Speed Delay

The speed delay component of travel time delay is affected by the temporary speed limit and by the capacity conditions of the work zone. As a base line, the initial travel time (before construction work) should be computed or measured. Most roadways that operate at less than capacity, the initial travel time may be estimated by dividing the segment length by the existing posted speed limit. It should be recognized, however, in areas where recurring congestion is present, the existing travel time should be inclusive of recurring congestion due to near or at capacity operations.

Measured operating speed may be available on RITIS. Existing travel time runs should be completed to verify the existing conditions.

The proposed posted temporary speed limit should be considered the maximum operating speed during construction activities. It is important to consider the presence of special reduced speed limits in Michigan, such as the “45 MPH Where Workers Present” condition. The actual operating speed within a work zone may still be less than the posted work zone speed depending on the capacity conditions and worker location.

The relative nearness of the traffic demand to capacity is related using the V /C ratio. The V/C ratio may be used as a performance measure for the work zone operations.

The speed delay attributable to the work zone can be computed as the difference in the travel time during construction and before construction. The following formula may be used to compute speed delay:

INSERT EQUATION
Where:
t = speed delay (min)
D = work zone length, from reduce speed signing to return to existing roadway speed (miles)
U_wz = operating work zone speed (mph)
U_i = existing posted or operating speed before construction (mph)

D. Queue Delay

A queue is an accumulation of vehicles for each time period in which the arrivals, or demand, exceeds the departures, or capacity. The queue delay component of travel delay is estimated only when the demand exceeds capacity and a queue forms. When the demand traveling through the work zone is less than or equal to capacity, no queue is formed and no queue delay is encountered.

Spreadsheet tools or software are helpful in performing the computation of queue delay, since the accumulation and dissipation of queues may occur across multiple time periods. The delay encountered at a queue is estimated using the volume of vehicles accumulated in the queue and the capacity at which the queue is serviced.

The following formula can be used to compute the delay time for a vehicle at the back of the queue:

INSERT EQUATION
Where:
t = queue delay (min)
Q = number of vehicles in queue (# of veh)
C_a = adjusted mainline capactity (vph)

Example 3.3: Urban Freeway, Queue Delay

Building from Example 3.2, the demand on this urban freeway is 3000 vph. The work zone is 5 miles long and the existing posted speed is 70 mph. During Construction the work zone will be signed for:

45 MPH

Where Workers Present.

Calculate the queue delay.

Solution 3.3 Urban Freeway, Queue Delay

The adjusted mainline capacity estimated earlier is 2798 vph. The accumulated queue for the time period (hour) is the demand that exceeds the capacity.

Q = 3000 - 2798 = 202 veh

The queue delay is:

t = 202 ∕ 2798 × 60 = 4.33 min

Table 33: Per Lane Capacity for Signalized Flow

Assumptions for Table 3-3: 5% trucks, level terrain, urban area
*Assumes no left turns through opposing traffic from lane analyzed. Left turns with opposing traffic merit special consideration. If no opposing traffic exists, left turns are treated as right turns. For example: no opposing traffic, 30% rights & 20% lefts, use table values for 50% turns.

Lane Characteristics			No Turns		50% Turns*		100% Turns* or Right-turn lane		U-turn crossover
Lane width (ft)			12	10	12	10	12	10	N/A
Green Time	40%	Unrestricted	620	550	560	500	520	450	530
		Restricted	580	510	520	460	480	420	490
	50%	Unrestricted	850	750	770	680	710	620	670
		Restricted	800	700	730	640	670	590	630
	60%	Unrestricted	1040	910	940	830	870	760	810
		Restricted	970	850	880	780	810	710	760

3.01.02 Traffic Regulator Operation

(Updated 12/3/2020) Traffic regulator operations, on the mainline traffic with alternating flow, should be evaluated based on travel time delay. The travel time delay calculation is determined using the Construction, Congestion, Cost Software (CO3) Flag Spreadsheet. The capacity of a traffic regulator operation is directly related to the posted work zone speed, volume of traffic, and the length of the traffic regulator zone.

The chart and graphs provided in Appendix B: Mobility Analysis Tools present capacity during traffic regulating on two-lane roadways. Once this determination is made, it can be used in the CO3 analysis to evaluate the travel time delay associated with work zone lengths. The value, as determined by the chart and the CO3 analysis, should be specified in the TMP. The length of the work zone and other design restrictions must also be placed in the Special Provision for Maintaining Traffic.

Example 3.4: Traffic Regulator

A seven mile milling and resurfacing construction project is scheduled on a two lane, two-way portion of M-44. Construction operations allow for traffic regulator control to maintain one lane, alternating during working hours, and two lanes open during non-working hours. The existing posted speed is 55 mph and the work zone will be signed for 45 mph.

Calculate the design hour volume (DHV) and delay for a traffic regulator set up.

Solution 3.4: Traffic Regulator

Traffic count information for this location can be found at MDOT TDMS. The TDMS data indicates that the AADT is 5,950 vehicles for this segment. This traffic volume is cumulative of both directions of travel. If a breakdown of hourly traffic is available it must be used or the AADT number needs to be converted using the design hour volume (DHV) in vph.

Multiply the AADT by the DHV % to obtain the DHV. TDMS indicates that the DHV% for this segment is 12%:

(5,950 veh) × (12% DHV) = 714 vph

Use the table for Appendix H. Capacity in Traffic Regulator Zones to determine the maximum allowable length of the work zone given the 45 mph speed limit. The design hour has 714 vph. Utilizing the table for Appendix H. Capacity in Traffic Regulator Zones traffic regulation operation will result in delays of 10 minutes or less (714 is less than 725 which is capacity at 2.0 miles). Confirm hourly volume calculation with hourly traffic volumes if available.

The work zone length should be further evaluated using the CO3 Flagging to confirm travel time delays do not exceed 10 minutes. The allowable length of the lane closure must be specified in the Traffic Restrictions in the Special Provision for Maintaining Traffic.