How to auto-accept Wi-Fi Direct connection requests in Android

It can be easily done with the help of Xposed framework. You just need to replace the single method inside one of android java classes (see the link from snihalani's answer). But of course to use Xposed your device must be rooted. The main idea can be expressed in the following code (using Xposed)

@Override
public void handleLoadPackage(LoadPackageParam lpparam) {
    try {
        Class<?> wifiP2pService = Class.forName("android.net.wifi.p2p.WifiP2pService", false, lpparam.classLoader);
        for (Class<?> c : wifiP2pService.getDeclaredClasses()) {
            //XposedBridge.log("inner class " + c.getSimpleName());
            if ("P2pStateMachine".equals(c.getSimpleName())) {
                XposedBridge.log("Class " + c.getName() + " found");
                Method notifyInvitationReceived = c.getDeclaredMethod("notifyInvitationReceived");
                final Method sendMessage = c.getMethod("sendMessage", int.class);

                XposedBridge.hookMethod(notifyInvitationReceived, new XC_MethodReplacement() {
                    @Override
                    protected Object replaceHookedMethod(MethodHookParam param) throws Throwable {
                        final int PEER_CONNECTION_USER_ACCEPT = 0x00023000 + 2;
                        sendMessage.invoke(param.thisObject, PEER_CONNECTION_USER_ACCEPT);
                        return null;
                    }
                });

                break;
            }
        }
    } catch (Throwable t) {
        XposedBridge.log(t);
    }
}

I tested it on SGS4 stock 4.2.2 ROM and it worked. I guess the same could be done with the help of Substrate for android.

Based on the comments, do you really need to connect to the devices if you just want to track and log the vehicles around you ?

I don't know the scope of the project, but you could simply use the WifiP2pDeviceList that you get when you request the peers in the WifiP2pManager. You could get the list of the devices (~= vehicles) around you and could log this.

Connection is useful if you want to send more detailed information I guess.

From my current understanding of the API, You cannot really accept connections automatically without user's intervention. You can initiate a connection, that doesn't require user intervention. If both of your devices are mobile devices, you will have to accept connection request on one end.

I have put this as a feature request in android project hosting. You can monitor their response here: https://code.google.com/p/android/issues/detail?id=30880