on . Posted in Civil Engineering Braking distance on downgrade or upgrade, abbreviated as $$B_d$$, is when the highway is on a grade.  The steeper the downgrade the longer it takes to stop.  The steeper the upgrade the shorter it takes to stop.

$$\large{ B_d = \frac{ V^2 }{ 30 \; \left( \frac{ a }{ 32.2 } \;\pm\; G \right) } }$$

$$\large{ B_d = \frac{ V^2 }{ 30 \; \left[ \left(\frac{ a }{ 32.2 }\right) \;\pm\; \frac{ G }{ 100 } \right] } }$$

Symbol English Metric
$$\large{ B_d }$$ = braking distance on grade  $$\large{ft}$$ $$\large{m}$$
$$\large{ a }$$ = deceleration rate $$\large{\frac{ft}{sec}}$$ $$\large{\frac{m}{sec}}$$
$$\large{ V }$$ = design speed $$\large{\frac{ft}{sec}}$$ $$\large{\frac{m}{sec}}$$
$$\large{ G }$$ = grade percent divided by 100 $$\large{dimensionless}$$

## AASHTO Exhibit 3-2 Stopping Sight Distance on grade

Design Speed Stopping Sight Distance
3% 6% 9% 3% 6% 9%
15 80 82 85 75 74 73
20  116  120  126  109 107 104
25  158  165  173  147 143 140
30  205  215  227  200 184 179
35  257  271  287  237 229 222
40  315  333  354  289 278 269
45  378  400  427  344 331 320
50  446  474  507  405 388 375
55  520  553  593  469 450 433
60  598  638  686  538 515 495
65  682  728  785  612 584 561
70  771  825  891  690 658 631
75  866  927  1003  772 736 704
80   965    1035   1121    859     817    782 