TedStixon

Answer: I honestly think within the context of the movie, we are just meant to assume that most units are either at the police station (given a high-profile terrorist suspect - the Joker - and many of his goons were just captured) or still at the scene of the massive chase that just occurred earlier in the night, since there was a lot of destruction or chaos. Thus, there simply wouldn't be any available units closer to Rachel's location. It's a little far-fetched, sure... but it's basic "suspension of disbelief." It's required dramatically for the plot to progress. (Similar to the fact that in the sequel, many of the Gotham City PD got stuck underground by Bane... it's a little far-fetched, but it works for the plot.) Batman was simply able to get to Dent's location faster since he's using the batpod, which is a bit of a technological marvel and can get around much quicker and easier than a police car.

TedStixon

Answer: This could be a plot hole, but I would posit: Given how corrupt/easily corruptible the Gotham police are, it's likely the Joker may have bribed/threatened/otherwise coerced the beat cops to a) steer clear of the areas where he was holding Dent and Rachel, or b) ignore the call to go to that location. You're right that Batman's tech is what allows him to reach Dent faster than Gordon et al. Reach Rachel; the Joker set it up that way, so that whoever Batman saves means the other one dies.

Answer: Batman showed up only a few seconds before the GPD, they arrived as the building blew.