use sensors. when the car is over the sensor, slow down the max speed of the car.
just set the isSensor property true on the fixture of the sensor shape. sensors will detect collision, but not collide physically.

i have created this for xCode, now. See this

http://varunxcode.blogspot.in/2013/04/xcode-car-race-with-box2d.html

just make that shape fixture definition sensor = true; like this

function createSensor(world, options)
{
options = $.extend(true, {
‘density’ : 1.0 ,
‘friction’ : 0.0 ,
‘restitution’ : 0.2 ,
‘linearDamping’ : 0.0 ,
‘angularDamping’ : 0.0 ,
‘gravityScale’ : 1.0 ,
‘type’ : b2Body.b2_dynamicBody
}, options);

var body_def = new b2BodyDef();
var fix_def = new b2FixtureDef;
fix_def.shape = new b2PolygonShape();
var vertices = new Array();
vertices[0] = new b2Vec2(3.24, 13.66);
vertices[1] = new b2Vec2(6.34, 14.50);
vertices[2] = new b2Vec2(7.50, 13.35);
vertices[3] = new b2Vec2(4.13, 13.00);
fix_def.shape.SetAsArray(vertices, 4);
fix_def.isSensor = true; // <————————————————-
world.CreateBody( body_def ).CreateFixture(fix_def);
world.SetContactListener(listener);
}
var listener = new Box2D.Dynamics.b2ContactListener;
listener.BeginContact = function(contact) {
// console.log(contact.GetFixtureA().GetBody().GetUserData());
}
listener.EndContact = function(contact) {
// console.log(contact.GetFixtureA().GetBody().GetUserData());
}

use .R -> .q, .R.col1 -> .q.GetXAxis() and .R.col2 became .q.GetYAxis()

Did you get any help in xCode? please let me know, if you get the code for xCode

i have resolved this problem. Just go in redraw_world function and write following code

context.clearRect(0, 0, 500, 300);//Area of your canvas
context.save();
//maxY is the max position by car (extreme top) same with maxX(extreme right) in b2vec2 coordinate. “wd” is canvas width, “ht” is canvas height, zoomLevel 1 by default (canvas zoom actually)
context.translate(((car.body.GetPosition().x*wd)/maxX)*zoomLevel,((ht)-(car.body.GetPosition().y*ht/19.18))*zoomLevel);

context.rotate(-car.body.GetAngle());

context.drawImage(carImage,-carImage.width/2, -carImage.height/2);//(((car.body.GetPosition().x*wd)/maxX)-carImage.width/2),((ht)-(car.body.GetPosition().y*ht/19.18)-carImage.height/2));

context.restore();