Why does the terminator leave the scene of the car crash?

Well, the obvious answer is for the drama, but analyzing the terminator's actions using the movie's logic, we can infer the reason behind its behaviour:

1. The terminator was damaged during the crash. We see in the next scene that its right arm is malfunctioning and that its left eyelid is permanently closed. It needs to repair its arm and remove said eyelid in order to see again. It goes to the police station only after repairing itself. We can assume it calculated that its chances to take down the officers (4 to 5 police cars with cops fully armed) were pretty low without the use of one arm and without depth perception.

2. It was running out of ammo. The terminator used a lot of ammo trying to take down Reese and Sarah in the chase. Later, when it repairs itself, it takes more weapons and ammo and goes to the police station.

We have seen in the movies that even if the T-800 is a walking tank, it is still vulnerable. If it had engaged in combat without full-combat capacity and with low ammo, and one of the cops decided to run it over with his police car once he saw the thing was a robot, it might have been taken down. When it attacks the police station, the terminator has the advantage, because the police aren't expecting a sudden attack into their headquarters. We see there is a lot of confusion and even having a building full of cops, only a few were able to hit it. Which leads us to another point:

3. The terminator is an infiltration unit. A thing that most people forgets is terminators aren't supposed to be demolition machines or assault troopers. Their first mission is to maintain a facade and keep it as long as possible to eliminate precise targets. We see that it does engage in full and open combat but only when there is a high chance to kill its target. Right after the crash, there were low chances to succeed, so the terminator fled from the scene. Even when it goes to the station, it first asks to see Sarah, trying to get to her in a way that doesn't draw too much attention. It is only after it can't get to her, and it knows she might be guarded by police for a long time (maybe even going to jail) that it takes the decision to assault the police station, and even then it does it in the quickest way possible. It doesn't go after each police officer. It cuts the lights off and tries to find and kill Sarah as fast as it can.

This is covered in the Frakes novelisation

• The Terminator has just suffered a substantial impact. Several systems went offline temporarily:

The police cruiser folded around the cyborg like a cheap accordion. Terminator’s vision dropped out first. Then all the systems in its microprocessor went offline due to the horrific impact to its hardened chassis.

• It's been damaged in the gunfight. Its arm is in a very poor state and it has "total occlusion" of the left eye. It needs to determine the extent of the damage caused:

A shadow among shadows, Terminator climbed with slow, patient steps up the fire escape to his second-story window. It avoided using its disfunctioning right wrist until it could determine the full extent of damage it had sustained in the initial combat.

It had taken the cyborg nearly an hour to get from the site of the crash back to its hotel room.

It had moved on foot for the first couple of miles, allowing the systems to come fully on line in order to assess their condition. Aside from the wrist, there was almost total visual occlusion of the left eye. The eye itself seemed to be functioning properly. It was the surrounding tissue that was hindering performance.

• It knows where its targets are likely to be, so there's no desperate urgency in getting to them:

Starting with the As, Terminator rapidly dialed every police station in Los Angeles until it reached Rampart Division.

Now it was time to move out. The target was waiting.

It seems to have been a plot contrivance written into the film to extend its length.

While the Terminator is indeed an infiltration unit, it’s core mission was to eliminate Sarah Connor to ensure that her son, John Connor, the leader of the human resistance to Skynet in the future is never born. Several times throughout the film, its actions however lead the discerning viewer to believe that it fails to understand this mission.

Some examples:

1. When it determines where Sarah Connor lives, it doesn’t simply wait at or near her apartment for her to return to kill her even that would be the most logic course of action.
2. Even though the film makes it clear that Skynet (and thus the Terminator) has no idea of exactly which Sarah Connor is which, it no longer hunts additional Sarah Connors after the events at the police station and concentrates solely upon the film’s protagonist.
3. The Terminator doesn’t simply break Sarah Connor’s neck when it discovers her in the bar when it is assaulted by Kyle Reese. The Terminator could easily done this and then left largely undetected, yet slowly raises a laser-equipped firearm to shoot her in the head and is stop by Reese. The Terminator also does not shoot Connor in the torso even though using the weapon that it was armed with would have injured her enough to kill her.
4. When the Terminator assaults the police station, it engages the police inside in a firefight that slows its ability to locate Connor, when it could have simply walked through the station and located her. The police, as demonstrated, had no weapons available which could slow or halt the Terminator and it could have easily
5. During the vehicular accident, the Terminator could have easily (again) walked to Reese and Connor, killed them without using a weapon and left the area. It chose its own “survive’ over its core mission, even though this could have meant that particular Sarah Connor could have escaped.

So while its actions make logical sense in the film’s universe (killing Sarah Connor would have ended the film prematurely) they make no logical sense when the film is looked at objectively, making the reason a plot contrivance that borders upon being a plot hole.

I feel there are two possible answers: like someone mentioned, maybe he wanted to be able to survive without being captured to continue hunting other Sarah Connors (unlisted in phonebook). But that doesn't make sense seeing how he took on the police station, which means he would've been injured enough to be able to only escape

as for the above additional "illogical" moves:

1. the Terminator didn't have to wait for Sarah if he knew where she was - esp as she might go into police custody
2. the Terminator has no other option but to pursue the next Sarah Connor. no point in looking for others whose existence is questionable
3. I felt the slowness of the machine was why he chose the gun. sure he ripped that dudes heart out in the beginning, but a gun is quicker in case Sarah tried to run.
4. I think shooting the police is easier than walking through them - if Reese did damage to him, so could the police. he's not made of adamantium