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***