Missing girls *spoilers*

Since the movie appears to have take place in Nebraska (the address of Bob's house on the letter his brother sent was to a NE address), I'm going to venture that he lived along the I-80 corridor. There are lots of fairly small communities scattered along I-80, and a few decent sized cities...most of which within an hour or so. Plus, depending on your cell carrier, there are lots of dead air spaces out there. It wouldn't be difficult to pick up someone from a town or city along the interstate, and get off into the country. Once you were on the backroads, it would be very difficult for anyone to locate you, even with a cell.

I don't doubt they had clues, but he was so random it would be hard to establish a pattern, other than them being female and young. Plus, I think that alot of them didn't realize the danger till it was too late, and I also got the impression that once he was off the main roads, he either locked them in the trunk or knocked them unconscious.

Bundy is a good comparison as he has a similar MO to Bob, but shows just how silly Bob's serial killing was:
1. Bundy known killings before he was first caught cover a period of one and a half years. Bob keeps killing for at least ten years during the time he holds Rabbit, and seems to have been killing for some time before he catches Rabbit.
2. Bundy was highly mobile. His longest killing period in one place was 7 months in Washington and Oregon. After that he moved to Utah, Colorado and Idaho. Bob never moves and always seems to abduct girls more than a few hours drive from his house.
3. After six months and six abductions by Bundy around the Pacific Northwest area there was panic. Nobody seems to notice Bob's abductions beyond small aritcles in the newspaper.
4. Bundy drove a nondescript Beetle that nobody looked inside until a policeman pulled him over and caught him. Bob drives a taxi: it's by design a high-profile and noticeable car, his details will be on record, and people who he doesn't murder are constantly getting into his taxi and fail to notice any evidence.
5. Bundy was identified as a possible suspect after a few months but was dismissed as he was a clean-cut law student. Bob on the other hand looks and acts just like you'd expect a serial killer to.
6. Bundy lived in lived in a house. Bob lives in a serial-killer-style bungalow lair with no windows and a thick metal door in the middle of a field but within sight of a road. Nobody saw that house and thought it suspicious?
7. Bundy bludgeoned his victims before or immediately after they got into his car. Bob just leaves them panicking in the back of the car until he gets home. None of them was able to signal a passing car or pedestrian for help? Nobody noticed a taxi that regularly had a screaming woman in the back seat over a time period of more than ten years?

Realistically, it wouldn't have taken more than a few abductions before a witness turns up who can confirm that they last saw the abductee getting into a taxi. The police would want to get in touch with the taxi driver, at least to eliminate them from their enquiries, and when no-one comes forward the police get hold of a list of taxi drivers and it's a matter of days before someone comes round to interview Bob and notices that he's an obvious serial killer type or if he's not home that is house is obviously designed to prevent people escaping.

Bob received money from the other cab customers, which really does add to the plot hole of the cab company/police not becoming suspicious.

Also his brother may very well been supplying him with money.