Reducing Lane and Shoulder Width to Permit an Additional

Lane on a Freeway

June 7,2016

Karen Dixon, Ph.D., P.E.Kay Fitzpatrick, Ph.D., P.E.

Raul Avelar, Ph.D., P.E.

Presentation Overview

• Background Information

• Operational Analysis

• Safety Analysis

• Questions

2

Typical Lane and Shoulder Widths

Agency Lane Widths

Shoulder Widths

AASHTO 12 feet Right: 10 feetLeft: 4 feet (2 through lanes), 10 feet (≥ 3 through lanes)

TxDOT 12 feet Same as AASHTO

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Operational Effects – Lane Width

Average Lane Width

(feet)

Reduction in FFS (mph)

≥ 12 0.0

≥ 11 – 12 1.9

≥ 10 – 11 6.6

FFS Adjustment based on Freeway Lane Width

Source: Highway Capacity Manual, Exhibit 11-8, p. 11-11 (TRB, 2010)

4

Operational Effects – ShoulderWidth / Lateral Offset

Right-Side

Lateral Clearance

(feet)

Lanes in One Direction

2 3 4 ≥ 5

≥ 6 0.0 0.0 0.0 0.0

5 0.6 0.4 0.2 0.1

4 1.2 0.8 0.4 0.2

3 1.8 1.2 0.6 0.3

2 2.4 1.6 0.8 0.4

1 3.0 2.0 1.0 0.5

0 3.6 2.4 1.2 0.5

FFS Reduction (mph) for Freeway Right-Side Lateral Clearance

Source: Highway Capacity Manual, Exhibit 11-9, p. 11-12 (TRB, 2010)

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Operational Effects – Influential Factors or Performance Measures to Note

• Physical dimensions (including minimum values present) and bounding condition (Barrier or Guardrail? Sideslope?)

• Heavy vehicle volume

• Balance of lane width (and distribution) with companion shoulder width

• Free flow speed

• Lane capacity and lane location

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Safety Effects – Lane & Shoulder Width (3% to 11% increase in crashes)

Treatment

Traffic Volume

AADT (vehicles /

day)

Crash Type

(Severity)CMF

Std.

Error

4 to 5 lane

conversion

79,000 to 128,000

one direction

All types (All severities) 1.11 0.05

All types (Injury and Non-injury

tow-away)1.10 0.07

All types (Injury) 1.11 0.08

5 to 6 lane

conversion

77,000 to 126,000

one direction

All types (All severities) 1.03 0.08

All types (Injury and Non-injury

tow-away)1.04 0.10

All types (Injury) 1.07 0.10

Base Condition: 4 or 5 lanes (12 feet wide) depending on initial roadway geometry

Crash Effects of Adding Lanes by Narrowing Existing Lanes and Shoulders

Source: Adapted from the Highway Safety Manual, Table 13-5, p. 13-10 (AASHTO, 2010)

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Safety Effects – Lane Width for Texas Urban Freeways (Increase of 7%)

Lane Width

(feet)

Number of Through Lanes (in one direction)

2 3 4 5

10 1.07 1.06 1.06 1.07

10.5 1.06 1.05 1.05 1.05

11 1.04 1.03 1.03 1.03

11.5 1.02 1.02 1.02 1.02

12 1.00 1.00 1.00 1.00

Base Condition: Lane width of 12 feet

Lane Width CMF Values for Texas Urban Freeways (Injury + Fatal Crashes)Source: Roadway Safety Design Workbook

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Safety Effects – Right Shoulder for Texas Urban Freeways (Increase of 3% to 6%)

Shoulder

Width (feet)

Number of Through Lanes (in one direction)

2 3 4 5

≤ 6 1.06 1.04 1.03 1.03

7 1.05 1.03 1.02 1.02

8 1.03 1.02 1.01 1.01

9 1.02 1.01 1.01 1.01

10 1.00 1.00 1.00 1.00

11 0.99 0.99 0.99 0.99

12 0.97 0.98 0.99 0.99

Base Condition: Outside shoulder width of 10 feet

Outside (Right) Shoulder Width CMF Values for Texas Urban Freeways (Injury + Fatal Crashes)

Source: Roadway Safety Design Workbook

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Safety Effects – Left Shoulder for Texas Urban Freeways (Increase of 7% up to 16%)

Shoulder Width

(feet)

Number of Through Lanes (in one direction)

2 3 4 5

0 1.07 1.16 1.14 1.15

1 1.05 1.14 1.12 1.13

2 1.04 1.12 1.11 1.12

3 1.02 1.11 1.09 1.10

4 1.00 1.09 1.08 1.08

5 0.98 1.07 1.06 1.07

6 0.97 1.06 1.05 1.05

7 0.95 1.04 1.04 1.04

8 0.93 1.03 1.02 1.03

9 0.92 1.01 1.01 1.01

10 0.90 1.00 1.00 1.00

Base Condition: Inside shoulder width of 4 feet for 2 lanes in one direction, and 10 feet for ≥

3 lanes in one direction.

Inside (Left) Shoulder Width CMF Values for Texas Urban Freeways (Injury + Fatal Crashes)

Source: Roadway Safety Design Workbook

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Operational Analysis

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Source of Speed Data

• TxDOT sensors – per lane data

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Sensor in Dallas

Number of Speed Sites

13

CityLanes with 11-ft Width

Lanes with 12-ft Width

Total Number of Lanes

Dallas 52 30 82

Houston 16 47 63

San Antonio

15 43 58

Total 83 121 204

Locations of Speed Sites

14

San AntonioHoustonDallas

Speed Site Variables (slide 1 of 3)

• Number of general purpose lanes on freeway (per direction)

• Posted speed limit (mph) – 60, 65, or 70 mph

• Lane width (ft) – 11 or 12 ft

• Left shoulder width (ft)

• Right shoulder width (ft)

• Median width (ft)

• Median type – grass or concrete barrier

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Speed Site Variables (slide 2 of 3)

• To right and left of the lane:

– Lane = another lane

– Shoulder = either right or left shoulder

– Speed change lane

– Buffer + pylons (separates managed lanes from general purpose lanes)

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Speed Site Variables (slide 3 of 3)

• Ramps

– Distance to next ramp – downstream (ft)

– Distance to next ramp – upstream (ft)

– Number of ramps within 1.5 miles – downstream

– Number of ramps within 1.5 miles – upstream

• Curvature – engineering judgment (yes/no)

– Horizontal

– Vertical

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Other Variables for Speed Study

• Speed – average operating speed per lane for 5-minute time period (mph)

• City – Dallas, Houston, or San Antonio

• Volume – 5 min/lane volume

• Natural light level – day or night

• Day of week – Wednesday or Saturday

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Speed Data Filters

• Removed all HOV lanes

• Removed lanes with widths > 12 due to being in a merging area

• Removed the following (served as surrogate to identify free-flow speeds)– Speeds < 50 mph

– 5-min volume > 250 veh

• Removed speed data that occurred within 30 minutes of sunrise or sunset

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Number of Speed Measurements

11-ft Lane Width 12-ft Lane WidthNumber of

LanesFrequency

Number of Lanes

Frequency

2 0 2 3,0053 108,715 3 110,5754 123,684 4 87,1195 108,296 5 125,903

All 340,695 All 326,602

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Speed Data Analysis Technique

• Perform cross-sectional evaluation on speed, volume, site characteristics

• Evaluate operational effects via statistical analysis:

– Objective: model operational speed to isolate its relationship with lane and shoulder width, after accounting for other influential variables

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Resulting Speed Model Variables

• Significant variables

– Volume

– Natural light level (day or night)

– Day of the week (Saturday or Wednesday)

– Use of the space neighboring the lane (To_Leftand To_Right)

– Median type (grass or barrier)

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Final Model for a Freeway LaneSpeed = 67.80

– 0.00018V25-min/lane (between 0 and 250 veh)

+ 4.60(if grass median, concrete barrier otherwise)

+ 2.21(if 12 ft lane width, 11 ft otherwise)

+ 3.62(if buffer + pylons to left)

+ 2.03(if shoulder to left)

– 3.89(if speed change lane to right)

– 4.39(if shoulder to right)

– 2.00(if night, daytime otherwise)

– 1.47(if Wednesday, Saturday otherwise)

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Identifies lane placement –only one of these conditions will apply

Examples of Predicted Speeds

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Speed Study Key Findings

• Lane width – 12 ft lanes are about 2.2 mph faster than 11 ft lanes (other variables held constant)

• Shoulder width – for this dataset, shows influence for left-most lane that are 11 ft wide (1.1 mph / 1 ft change in shoulder width)

• Freeway speeds are lower at night (2 mph) and higher on the weekend (1.5 mph, Saturday compared to Wednesday)

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Safety (Number of Crashes)

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Site Selection

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• Site = one direction (similar to operations)

• Define freeway segments to coincide with speed spot locations (when possible)

• Additional sites were also located

• No site included a ramp within its limits

Location of Safety Sites

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San AntonioHoustonDallas

Crash Data

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• Freeway crashes for years 2010 – 2013

• Data were disaggregated in 6-month periods to correctly account for changes in cross sections

• Unit of analysis is segment-years for each site:1 segment-period:

(length of segment)x(analysis period length)

Summary Statistics

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Variable Mean Min Max Total

Total Crashes 4.1 0 37 2202

KAB Crashes 0.8 0 6 407

Period Length (yr) 0.48 0.08 0.50 -

AADT (vpd) 152,163 200 281,450 -

Segment Length (ft) 1897.2 618 4510 -

Number of Lanes 3.8 2 6 -

All Lanes Width (ft) 45.2 24 73 -

Average Lane Width (ft) 11.8 10.8 12.5 -

Left Shoulder Width (ft) 9.1 1.3 22.9 -

Right Shoulder Width (ft) 10.3 2.0 14.8 -

Closest Downstream Ramp (ft) 1861.2 17 6938 -

Closest Upstream Ramp (ft) 1738.7 320 7170 -

N = 536

Resulting Crash Model Variables

AADT

Number of Lanes

Right Shoulder

Left Shoulder

Distance to upstream ramps

Distance to downstream ramps

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Final Model for Total Crashes

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NTotal= 1.0027 × L × AADT0.539

× e −1.0243 Ramp_Up_D −1.0877 Ramp_Dn_D

× e −0.0241 NLane×Lane_WAvg

× e −0.0735 R_Shld_W −0.0646 L_Shld_W

A negative sign indicates a reduction in crashes when these variables increase

Final Model for KAB Crashes

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NKAB= 0.0514 × L × AADT0.662

× e −1.5787 Ramp_Up_D −0.8659 Ramp_Dn_D

× e −0.0253 NLane×Lane_WAvg

× e −0.0956 R_Shld_W −0.0547 L_Shld_W

Safety Study Key FindingsCrashes vs. Lane Width

5% fewer KAB crashes at 2-Lane (2-L) freeways with 12 ft lane widths, compared to 2-L freeways with 11 ft lane widths

12% fewer KAB crashes at 5-Lane (5-L) freeways with 12 ft lane widths, compared to 5-L freeways with 11 ft lane widths

Similar trends occur for Total Crashes

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Safety Study Key FindingsCrashes vs. Number of Lanes

24% fewer KAB crashes per additional lane when the average lane is 11 ft

26% fewer KAB crashes per additional lane when the average lane is 12 ft

Similar trends for Total Crashes

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Safety Study Key FindingsCrashes vs. Shoulder Widths

5% fewer KAB crashes per additional foot of left shoulder width

9% fewer KAB crashes per additional foot of right shoulder width

Similar trends for Total Crashes

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Example Application of Models

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Example Problem #1What is the expected daytime operating speed on a weekday for a freeway lane with the following characteristics:

• Hourly volume = 1200 vph (100 veh/5-min),

• Number of lanes = 3,

• Lane width = 12 ft,

• Median type = concrete barrier, and

• Lane position of interest = center lane.

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Solution – Example #1

Hourly volume rate of 1200 veh/hr corresponds to 100 veh/5-

min = (1200 veh/hr)/(20 5-min/hr periods)].

Center lane in a 3-lane freeway section has a lane on both the

right and left.

Wednesday can be considered a representative weekday.

Speed = 67.80 – 0.00018(100)25-min/lane + 4.60(0) + 2.21(1) +

3.62(0) + 2.03(0) – 3.89(0) – 4.39(0) – 2.00(0) – 1.47(1) =

66.7 mph

Conclusion: An operating speed of approximately 67 mph can be

expected for the freeway lane.

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Example Problem #2

What is the expected total number of crashes and number of injury crashes for a freeway section with the following characteristics:

• Length= 0.5 miles

• Number of Lanes = 4 (average width of 12 feet each)

• AADT=150,000 vpd

• Right Shoulder Width = 10 ft

• Left Shoulder Width = 6 ft

• Distance to closest upstream ramp = 0.5 mi,

• Distance to closest downstream ramp = 1.0 mi

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Solution – Example #2

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𝑁𝑇𝑜𝑡𝑎𝑙 = 1.0027 × 0.5 × 150,0000.539 ×

𝑒 −1.0243 0.5 −1.0877 1.0 −0.0241 4×12 −0.0735 10 −0.0646 6 = 6.39

𝑁𝐾𝐴𝐵 = 0.0514 × 0.5 × 150,0000.662 ×

𝑒 −1.5787 0.5 −0.8659 1.0 −0.0253 4×12 −0.0956 10 −0.0547 6 =1.08

Conclusion: A total number of approximately 6 to 7 crashes,

including 1 injury crash, can be expected for the freeway location.

Example Problem #3

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𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐶𝑟𝑎𝑠ℎ𝑒𝑠

= e[ −2.53 ×10−2 × 55−48 + −9.56 ×10−2 × 8−10 + −5.47 ×10−2 × 1−6 ] = 1.33

Summary

• Speed predicted based on specific lane

• Adding lanes will result in:

– Increased capacity

• Narrowing lanes and shoulders will result in:

– Increased number of crashes and reduced speeds

• Methods can be used to minimize safety impacts by optimizing lane and shoulder width effects

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Questions?

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