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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