2001-03-19 Special Called Regular Meeting and Public Hearing

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" I tl, '\ " !t :1 !. I' " -j !I " .1 ~ . .4 '~ ., ,. .~ " .. ,", .... ... I ';, - ~:- ~. ~ !r e 8J e 106 Chap. 4 I Functional Circulation Systems the actual involvement rates may vary between rural and urban locations, the relative rates presented in Table 4-5 may be expected to be much more transferable. In this form, the data indicate that a vehicle traveling on an at-grade arterial at a speed 10 mph slower than the speed of the normal traffic stream is 180 times (20,000/110) more likely to be involved in an accident than a vehicle traveling at the same speed as the other vehicles in the traffic stream. A vehicle traveling 15 mph slower than the traffic stream has 90 times (20,000/220) the chance of being involved in an accident as a vehicle traveling 10 mph slower. While the relative ranges may be in considerable error, for any specific section of street or freeway, they clearly show that increased speed differentials result in substanti::llly increased accident potential. Thus, designs which produce small speed differentials (less than 10 or 15 mph) should be major criteria for the functional design of arterials. As will be presented in Chapter 5, all reasonable curb-return radii and throat-width combinations result in low-speed turn maneuvers and. hence. high speed differentials on arterial streets. Therefore. a right-turn bay (Figure 4-19) or a continuous right-turn lane (Figure 4-20) should be provided. GUIDELINES FOR THE LOCATION OF UNSIGNALlZED ACCESS Unsignalized direct access to arterial streets can be provided without seriously interfering with the movement function so long as careful attention is given to the location. spacing, movements permitted. and the design of such access. The same criteria should be applied to public and private access. Poorly designed access. or unrestricted access (Figure 4-21), results in functional obsolescence of the arterial as well as the private development. TABLE 4-5 Relative Accident-Involvement Rates ~~ ~ -t.6 Speed Differential (mph) 0 -10 .l.20 -30 At-grade arterials: Accident rate 100 220 720 5,000 Ratio, Oomph differential 1 2 6.5 45 10-mph differential 1 3.3 23 Freeways: Accident rate 30 100 600 2.000 Ratio, Oomph differential 1 3.3 20 67 1 Oomph differential 1 6 20 -35 20,000 ' Lf 0 ;a:::'J::r1 180 rv'1CfC> 45 'l I:!' I:!' -~-----------------~-- Figure 4-19 Right-turn bay. Provision of a right-turn deceleration lane will reduce the speed differential between turning vehicles and through traffic. It will also decrease the length of time during which the turning vehicle is causing severe turbulence in the traffic stream. Consequently, rear-end and side-swipe collisions between following cars are reduced. '~