Improving The Capacity Of An Existing Highway Section: Analysis And Recommendations

Your report is expected to answer the following questions with sufficient justification.
1. What is the existing LOS on this 1000m section of freeway?
2. Given that the anticipated rate of annual growth in traffic volume in the area is expected to be 18%, what will the LOS be in three years?
3. What are possible improvements that can be made to the existing freeway to delay capacity flow conditions for three years, given that the existing right-of-way cannot be expanded?

Traffic Characteristics of the Existing Freeway Section

Existing freeway section 1000 m = 1 km

Number of asphalt-grade lanes = 4 (2 in each direction)

Lane dimensions = 3.7m lanes with 0.3m right shoulder lateral clearance and 1.2m median with a concrete barrier

Total right-of-way = 21.8m

Interchange density = 1 per km

Design speed = 90 km/h (upgrade) and 1000 km/h (downgrade)

Peak hour volume = 1280 vehicles on a grade of 5%

5% trucks, 4% buses (it is assumed that the remaining 91% are personal cars)

PHF = 0.85

Existing right-of-way cannot be expanded

Anticipated annual growth rate in traffic volume = 18%

Peak hour volume:

Base year = 1280 vehicles per lane per hour (veh/ln/hr); total number of vehicles per hour = 1280 x 4 = 5120 veh/hr

First year =  = 6042 veh/hr

Second year =  = 7130 veh/hr

Third year =  = 8414 veh/hr

Existing LOS on the 1km freeway section

Level of service (LOS) is a qualitative measure that describes a freeway’s operating conditions. This measure is based on a variety of factors including: travel speed, delay, maneuverability, travel time and safety. LOS can be evaluated based on density of vehicles, volume/capacity (V/C) ratios, maximum service flow rates, and average travel speeds. The process of determining LOS starts by estimating free flow speed. The steps followed are as provided below [1]

Where V = peak hour volume, and V₁₅ = traffic volume during the peak 15-minute flow [2]

Estimating free flow speed of vehicles on the freeway:

Free flow speed (FFS) = BFFS – f_ID – f_LW – f_N – f_LC

Where BFFS = base free flow speed, f_ID = interchange density adjustment factor, f_LW = lane width adjustment factor, f_N = number of lanes adjustment factor and f_LC = right shoulder lateral clearance adjustment factor

f_LW = 0 (since lane width is >3.6m), f_LC = 4.8 km/h (obtained from right shoulder lateral clearance adjustment table), f_N = 7.24 km/h, f_ID = 8.05 km/h (obtained from Table 1, 2, 3 and 4 in the Appendix) [3].

FFS (upgrade) = 90 – 8.05 – 0 – 7.24 – 4.8 = 69.91 ≈ 70 km/h

FFS (downgrade) = 100 – 8.05 – 0 – 7.24 – 4.8 = 79.91 ≈ 80 km/h

Therefore average passenger car speed for upgrade and downgrade is 70 km/h and 80 km/h respectively.

Adjusting hourly volume to determine flow rate (passenger cars per lane per hour)

V_p =

V_p = passenger car equivalent flow rate over a 15-minute peak, V = peak hourly volume, N = number of lanes, PHF = peak hour factor, f_P = adjustment factor for driver population, and f_HV = adjustment factor for heavy-vehicles.

LOS of the Existing Freeway Section

V = 5120 veh/hr, PHF = 0.85, f_P = 1, and f_HV = (passenger car equivalent – E_T = 1.5 (level terrain)) (obtained from Table 5 and 6 in the Appendix).

 = 0.96

 = 1569 passenger cars per lane per hour (pc/hr/ln)

Density (upgrade) =  = 22.414 pc/km/ln

Density (downgrade) =  = 19.6125 pc/km/ln

LOS (upgrade) = LOS D

LOS (downgrade) = LOS D (0btaiined from Figure 1 in the Appendix)

Therefore existing LOS on the 1km freeway section is LOS D.

Peak capacity = base capacity x N (number of lanes in one direction) x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph) [1] 

Let’s use FFS = 80 km/h (downgrade); Base capacity = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 2 x 0.85 x 1 x 0.96 = 3588

Total volume = 2 x 1280 = 2560

Volume/capacity (v/c) ratio = 2560/3588 = 0.71

LOS of D means that traffic along the existing freeway section is approaching unstable flow. This reduces travel speeds (and increases travel time), limits maneuverability, and reduces comfort levels of drivers. Spacing between vehicles is about 50m and delays are created during minor incidents.   

The values of expected characteristics for LOS D are as shown below (obtained from Table 7 in the Appendix)

	Expected characteristics for LOS D	Existing characteristics for LOS D
Max. density (pc/km/ln)	26.25	22.414 (upgrade) 19.6125 (downgrade)
Min. v/c ratio	0.85	0.71 (<1)
Minimum speed (km/hr)	85.02	80
Max. service flow rate (pc/hr/ln)	1950	1569

LOS on the 1km freeway section in three years

The peak hour volume = 8414 veh/hr

 = 2578 pc/hr/ln

Density (upgrade) =  = 36.83 pc/km/ln

Density (downgrade) =  = 32.23 pc/km/ln

LOS (upgrade) = LOS E

LOS (downgrade) = LOS E

Therefore LOS on the 1km freeway section in three years will be LOS E. This shows a deterioration of traffic situation on the freeway section from the existing LOS of LOS currently.

Peak capacity = base capacity x N (number of lanes in one direction) x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph) = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 2 x 0.85 x 1 x 0.96 = 3588

Total volume = 2 x 2104 = 4208

Volume/capacity (v/c) ratio = 4208/3588 = 1.17

LOS of E means that the traffic along the existing freeway section will be unstable. The V/C ratio of 1.17 means that traffic volume is equal to capacity. This makes it difficult for drivers to maneuver in traffic. It becomes difficult for travel speeds to reach posted limits. Spacing between vehicles is about 38m and delays are created in case of any incident that will disrupt traffic. Comfort level of drivers is also very low.

Expected Characteristics for LOS D and E

Therefore the existing freeway will reach maximum capacity with no improvements close to the end of the third year.

Based on values obtained from Table 7 in the appendix, the following are the comparison of other important characteristics of a freeway whose LOS is E:

	Expected characteristics for LOS E	Real characteristics in 3 years’ time for LOS E
Max. density (pc/km/ln)	26.54	36.83 (upgrade) 32.23 (downgrade)
Min. v/c ratio	1	1.17 (>1)
Minimum speed (km/hr)	78.52	80
Max. service flow rate (pc/hr/ln)	2200	2578

Possible Improvements

Since the right-of-way of the existing freeway section cannot be expanded, there are two main improvement suggestions. These suggestions are as follows:

This option entails construction of a temporary overpass/flyover structure that will cross over the existing freeway section. The overpass should have two lanes (one in each direction) with a total right of way of 10.9m (half the total right-of-way of the existing freeway). In total, the freeway section will have six lanes (three in each direction) hence total traffic volume will be distributed equally among the 6 lanes.

Determining LOS for Option 1

Based on this option, the LOS of the existing freeway at the start and end of three years will be as follows:

LOS at the start of the 3 years

 = 1046 pc/ln/hr

Density (upgrade) =  = 14.943 pc/km/ln

Density (downgrade) =  =13.075 pc/km/ln

LOS (upgrade) = LOS C

LOS (downgrade) = LOS C

Peak capacity = base capacity x N x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph) = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 3 x 0.85 x 1 x 0.96 = 5381

Total volume = 3 x 1280 = 3840

Volume/capacity (v/c) ratio = 3840/5381 = 0.71

LOS at the end of the 3 years

 = 1719 pc/ln/hr

Density (upgrade) =  = 24.56 pc/km/ln

Density (downgrade) =  =21.4875 pc/km/ln

LOS (upgrade) = LOS D

LOS (downgrade) = LOS D

Peak capacity = base capacity x N x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph) = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 3 x 0.85 x 1 x 0.96 = 5381

Total volume = 3 x 2104 = 6312

Volume/capacity (v/c) ratio = 6312/5381 = 1.17

From the calculations above, it shows that constructing an overpass will help the existing freeway section have a LOS C and LOS D at the start and end of the three years respectively. This option will help reduce traffic congestion along the freeway section to some level. However, it is not the best option for this scenario considering the resources (especially time, personnel and money) required to implement it. To implement this option, a lot of money, time, professionals and other workers will be needed. Additionally, construction of the proposed overpass will also affect traffic movement along the existing freeway since construction will be happening at that particular section.

Possible Improvements

This option entails converting lanes of the existing freeway section into HOT (high occupancy toll) lanes. The lanes will be operational during the three years. By so doing, the number of solo drivers along the freeway section will significantly reduce because these drivers will have to pay in order to use the lanes. Only carpoolers, trucks and buses will be allowed to travel along the freeway section for free. The general impact of this option is that it will significantly reduce total volume of vehicles along the freeway section thus lowering volume/capacity ration since capacity of the lanes will remain the same. The HOT lanes have to be technologically advanced such that solo cars will be identified by transponder and the solo drivers’ accounts or credit cards will be automatically charged. The solo drivers will not have to pay cash at the tollbooths. The efficiency of this option has been proven in some of the major cities across the world. One of such projects is the I-25 South Corridor through the T-REX area where HOT lanes were adopted to reduce congestion along the highway [4]. The main objective of this option is to discourage people from driving their personal cars and instead encourage them to use efficient, affordable, additional and effective transit, carpools and vanpools. This ultimately give travelers other travel options, optimizes use of the existing freeway and reduces traffic volume along the freeway section.

Determining LOS for Option 2

Let’s assume that introducing HOT lanes will reduce traffic volume in one direction by 60% (from 1280 veh/hr to 512 veh/hr).

First year =  = 605 veh/hr

Second year =  = 714 veh/hr

Third year =  = 843 veh/hr

FFS remains 70 km/h (upgrade) and 80 km/h (downgrade)

LOS at the start of 3 years

Total traffic volume = 4 x 512 = 2048 veh/hr

 = 628 pc/hr/ln

Density (upgrade) =  = 8.97 pc/km/ln

Density (downgrade) =  = 7.85 pc/km/ln

LOS (upgrade) = LOS A

LOS (downgrade) = LOS A (obtained from Figure 1 in the Appendix)

Peak capacity = base capacity x N (number of lanes in one direction) x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph)

Let’s use FFS = 80 km/h (downgrade); Base capacity = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 2 x 0.85 x 1 x 0.96 = 3588

Total volume = 2 x 512 = 2048

Option 1 - Temporary Overpass/Flyover Structure

Volume/capacity (v/c) ratio = 2048/3588 = 0.57

At the start of the three years, the LOS of the freeway section will be A. This is manly characterized by free-flow operation where vehicles will move above or at posted speed limits. Vehicles will be able to shift from one lane to another very freely. The average spacing between vehicles will also be great (about 167 m) and drivers will have greater levels of psychological and physical comfort. Point breakdowns or incidents’ effects will also be absorbed very easily. With a V/C ratio of 0.57, it also means that the capacity of the freeway section will be greater than the traffic volume along this section.

LOS at the end of 3 years

Total traffic volume = 4 x 843 = 3372 veh/hr

 = 1034 pc/hr/ln

Density (upgrade) =  = 14.77 pc/km/ln

Density (downgrade) =  = 12.925 pc/km/ln

LOS (upgrade) = LOS B

LOS (downgrade) = LOS B (obtained from Figure 1 in the Appendix)

Peak capacity = base capacity x N (number of lanes in one direction) x PHF x f_P x f_HV 

Base capacity = 1700 + 10 FFS (mph)

Let’s use FFS = 80 km/h (downgrade); Base capacity = 1700 + 10(49.71) = 2198

Peak capacity = 2198 x 2 x 0.85 x 1 x 0.96 = 3588

Total volume = 2 x 843 = 1686

Volume/capacity (v/c) ratio = 1686/3588 = 0.47

At the end of the three years, the LOS of the freeway section will be B (this is due to the anticipated 18% annual increase in traffic volume in the area). This is manly characterized by relatively free-flow operation where vehicles will maintain posted speeds. Maneuverability of vehicles will be slightly restricted and the average spacing between vehicles will be about 100 m. Drivers will also have greater levels of psychological and physical comfort. Point breakdowns or incidents’ effects will still be absorbed easily. With a V/C ratio of 0.57, it also means that the capacity of the freeway section will be greater than the traffic volume along this section.

In general, option 2 will maintain traffic volume below the freeway section’s capacity. This will reduce travel time (because vehicles will move at or above the posted speed limits), improve safety and comfort of drivers and passengers, and reduce cost of travel. It will also have other impacts such as reducing air pollution in the air due to significant decrease in number of vehicles travelling along this section.

Option 2 is also a comparatively low-cost option that can increase the number of people using the freeway and reduce traffic congestion. It also takes less resources (time, money and personnel) to implement. Most importantly is that the revenue collected from the tolls can be used to finance other infrastructure projects including the planned construction of a new freeway section. The revenue can also be used to finance construction of facilities that will upgrade and expand walking and biking infrastructure, and improve access to nearby transit. Therefore the recommended improvement to relieve congestion on the 1km existing freeway section while constructing the new freeway section is to adopt a strategic traffic management system, which is introducing HOT lanes along the freeway. With this improvement, LOS on the existing freeway at the start and end of the three years will be LOS A and LOS B respectively. Volume/capacity ratio on this section is also less than 1 (0.57 and 0.47) through the period.

References

Federal Highway Administration, "Appendix N: Procedures for Estimating Highway Capacity," U.S. Department of Transportation, 30 June 2017. [Online]. Available: https://www.fhwa.dot.gov/ohim/hpmsmanl/appn1.cfm. [Accessed 21 March 2018].

J. Li, L. Han and C. Chen, "Impact of Data Resolution on Peak Hour Factor Estimation for Transportation Decisions," Central European Journal of Engineering, vol. 3, no. 4, pp. 732-739, 2013.

Transportation Research Board, Highway Capacity Manual, Washington, DC: National Academies of Science, 2016.

M. Salisbury, "How to Improve I-25 Traffic Without Spending Big Bucks," Southwest Energy Efficiency Project (SWEEP), 14 July 2017. [Online]. Available: https://www.swenergy.org/how-to-improve-i-25-traffic-without-spending-big-bucks. [Accessed 21 March 2018].