9. Traffic Engineering (Capacity Analysis and Transportation Planning) Flashcards
What is capacity analysis?
How much traffic a given transportation facility can accommodate (quantitative measure of facility)
What is level of service?
How well the present traffic situation is on a given transportation facility (qualitative measure of facility) (i.e., quality of a given flow rate)
What is the base condition of a freeway or multilane highway and its criteria?
The full capacity of the roadway segment is achieved under good weather, good visibility, no incident/accidents, no work zones, and no pavement deterioration that affects operations.
Includes:
- No heavy vehicles
- Drive population composed of regular/familiar users of the facility
- 12-ft lanes width and adequate lateral clearances.
- Capacity under based conditions varies with Free Flow Speed (FFS).
In general, what is Free Flow Speed (FFS)?
- the mean speed of passenger cars measured during period of low to moderate flow (up to 500pc/hr/ln). (i.e., average speeds are constant in this range of flow rates).
Describe the speed-flow curves for uninterrupted flow on basic freeway and multilane highways segments?
- Constant Speed Range: the range of flow rates (pc/hr/ln) where the speed is constant. From flow rate 0 to BP; speed = FFS.
- Decreasing Speed Range: speed decreases from FFS parabolically from BP to the capacity.
- Capacity (c): occurs when the traffic stream density, D = 45 pc/mi/ln.
What is Freeway - FFS?
a theoretical speed when the density and flow rate on a FREEWAY are both zero. Expect to prevail at flow rates between 0 and 1000 pc/hr/ln
Sensitive to the following:
- Lane Width
- Lateral Clearances
- Total Ramp Density (most critical)
What is Total Ramp Density (TRD) [Freeway]?
is the average number of on-ramps, off-ramps by direction per mile. Applies to a 6-mile segment of freeway (3 miles in each direction of the study). i.e., total number of ramps located between 3-miles upstream and 3-miles downstream of the midpoint of the segment divided by 6.
Equation for FFS for FREEWAY segment?
HCM Vol. 2 - Equation - 12-2; FFS = BFFS - f_LW - f_RLC - 3.22TRD^0.84
- f_lw = lane width adjustment
- f_rlc = right side lateral clearance
Equation for FFS for MULTILANE segment?
HCM Vol. 2 - Equation 12-3; FFS = BFFS - f_LW - f_TLC - f_M - f_A
- f_TLC = total lateral clearance adjustment
- f_M = median type
- f_A = access point density
What is the default FFS for FREEWAY?
FFS = 75.4 mph
What is the default FFS for MULTILANE segment?
Design speed (if available) OR the posted/statutory speed limit: +5 mph for speed limits >= 50 mph and +7 mph for speed limits < 50 mph.
Process for solving Freeway/Multilane problems
- Calculate FFS: FREEWAY - Equation 12-2; MULTILANE - Equation 12-3
- Check for FFS adjustments: Exhibit 11-21 (FREEWAY only); otherwise 1.0 for both freeway and multilane.
- Calculate flow rate: Equation 12-9
- Calculate breakpoint volume: [1,000 + 40 x (75 - FFS_adj)] CAF^2 (1400 pc/hr/ln always for MULTILANE)
- Check V_p less than or equal to BP
- If V_p less or equal to BP then use FFS_adj in density equation, otherwise FFS_adj must be adjusted using Equation 12-1
- b. Calculate capacity, c: Equation 12-6 - c =2,200 + 10 x (FFS_adj - 50) and c_adj = c x CAF
- Calculate density: Equation 12-11: D=v_p/S
- Check Exhibit 12-15 for LOS
What factors affect the operation of a weaving segment?
- Length: distance between the merge and diverge
- Width: number of lanes within the weaving segment
- Configuration: the way the entry and exit lanes are aligned.
How to calculate density in a weaving segment?
D = (V/N)/S
where:
V = sum of flow rates for weaving and non-weaving
N = no. of lane within the weaving segment
S = average speed of all vehicles in weaving segment; Equation 13-22
What HCM chapter covers freeway merge and diverge segments?
Chapter 14
What is the max capacity of a sidewalk?
23 p/min/ft
What is the effective walkway width?
W_E = W_T - W_O
where:
W_T = total walkway width
W_O = sum of fixed objects effective widths and shy distances of linear features
What is the effective length of a fixed object?
Assumed to be 5x the object’s effective width.
What is the default pedestrian speed?
3.5 fps or 210 fpm
What are the three accident categories?
- Property damage only
- Personal injury
- Fatalities
What is the accident ratio for intersection in RMEV?
R = (no. of accidents x 10^6)/[(ADT)(No. of years)(365)]
What is the accident ratio for route segment in Hundred Million Vehicle Miles?
R = (no. of accidents x 10^8)/[ADT)(No. of years)(365)(L)]
What is the conservation of momentum equation?
(M_v)(V_0) = (M_v + M_1) x V_1
What is saturation flow at a signalized intersection?
the max throughput for the signalized intersection approach, if the approach was given a green light for a full hour. (S_h = saturation headway which is the min time headway, 2 sec.)
