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 vplph

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: