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110 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2
SCAG Arterial Speed Study
60
50
40
Mean Speed (mph)
30
20
10
y = 43.663e 1.5385x
R2 = 0.3047
0
0 0.2 0.4 0.6 0.8 1.0 1.2
One-Hour Volume/Capacity Ratio
FIGURE 1 Hourly speedflow observations.
TABLE 1 Speed Survey Site Characteristics
Lanes Speed
Length Facility Area (both Signals Limit ADT
Street From To (miles) Type Type dir.) (#) (mph) (2-way)
1st St. Ford Blvd. Gage Ave. 0.90 3 4 4 4 35 19,300
Aviation Blvd. W 120th St. W 135th St. 0.99 2 4 4 4 40 33,800
Beverly Blvd. Robertson Blvd. La Cienega Blvd. 0.44 2 2 4 4 40 34,700
Lincoln Blvd. Fiji Way Venice Blvd. 1.43 2 3 6 7 3540 54,600
San Vicente Blvd. Curson Ave. Hauser Blvd. 0.64 2 2 6 4 3540 39,900
Sunset Blvd. N La Brea Ave. N Cherokee Ave. 0.51 2 3 6 4 40 33,200
Verdugo Rd. Colorado Blvd. N Shasta Circle 0.83 3 5 4 4 3540 16,000
Western Ave. W 111th St. W 120th St. 0.68 2 4 46 4 35 21,300
Note: Facility type: 2, principal arterial; 3, minor arterial. Area type: 2, central business district; 3, urban business district; 4, urban; 5, suburban.
sequently, it was necessary to identify data points when where
the counted volume did not equal the demand and drop
these points from the data set. S average link speed (mph or km/h),
S0 free-flow link speed (mph or km/h),
X v/c ratio,
CANDIDATE SPEEDFLOW EQUATIONS a 0.15, and
b 4.
Several candidate speedflow equations might be fitted
to the observed data. Table 2 describes several candi- The BPR equation was originally fitted to 1965 Highway
dates. The first five candidates--linear, logarithmic, Capacity Manual freeway speedflow data (1). Since then
exponential, power, and polynomial--are standard additional research has indicated a less significant effect
mathematical functions commonly used in data analysis. of v/c ratio on mean speeds until capacity is reached (see
The last two equation forms--Bureau of Public Roads Exhibit 13-4 of the 2000 Highway Capacity Manual).
(BPR) and Akcelik--are unique to travel time and delay The Akcelik equation was derived by Akcelik from the
analysis. The BPR equation has been the traditional steady state delay equation for a single-channel queuing
method for predicting vehicle speed as a function of vol- system. He derived the following time-dependent form:
umecapacity (v/c) ratio in travel demand models.
L
S0 S= (1)
S=
1 + a ( X )b L / S0 + 0.25T ( x - 1) + ( x - 1)2 + 8
cT
Jx
